Transistor, method of manufacturing transistor, and display device using the same

ABSTRACT

A transistor in an embodiment includes an oxide semiconductor layer on a substrate, the oxide semiconductor layer including a first region and a second region, a first gate electrode including a region overlapping the oxide semiconductor layer, the first gate electrode being arranged on a surface of the oxide semiconductor layer opposite to the substrate, a first insulating layer between the first gate electrode and the oxide semiconductor layer, and a first oxide conductive layer and a second oxide conductive layer between the oxide semiconductor layer and the substrate, the first oxide conductive layer and the second oxide conductive layer each including a region in contact with the oxide semiconductor layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 16/577,044 filed on Sep. 20, 2019, which claims the benefit of priority from the prior Japanese Patent Application No. 2018-180487 filed on Sep. 26, 2018, the entire contents of which are incorporated herein by reference.

FIELD

An embodiment of the present invention relates to a transistor including an oxide semiconductor, a method of manufacturing the same, and a display device including a pixel that includes a transistor including the oxide semiconductor.

BACKGROUND

An active matrix display device includes a display element and a transistor driving the display element that are provided in each of pixels thereof. Usable as the display element is a liquid crystal element including a pair of electrodes and a liquid crystal layer provided between the pair of electrodes or an organic electroluminescence element (hereinafter, referred to as an “organic EL element”) including a cathode electrode, an anode electrode and a layer that is provided between the cathode electrode and the anode electrode and contains an organic electroluminescence material. The transistor may be a thin film transistor including components formed of an amorphous silicon semiconductor or a polycrystalline silicon semiconductor. Recently, a thin film transistor including components formed of an oxide semiconductor is also used.

An amorphous silicon semiconductor film may be easily formed on a substrate having a large area size, but has a problem of having a low field effect mobility when being used to manufacture a thin film transistor. By contrast, a thin film formed by use of a polycrystalline silicon semiconductor has a high field effect mobility, but has the following problem. A polycrystalline silicon semiconductor requires a crystallization step such as laser annealing or the like, and therefore, is not easily formed into a uniform film on a substrate having a large area size. This causes the threshold voltage to be varied. A thin film transistor manufactured by use of an oxide semiconductor has a field effect mobility higher than that of a thin film transistor manufactured by use of an amorphous silicon semiconductor film. In addition, a thin film transistor manufactured by use of an oxide semiconductor is more easily formed on a substrate having a large area size than a thin film transistor manufactured by use of a polycrystalline silicon semiconductor, and has an advantage of not requiring a crystallization step. However, an oxide semiconductor film has the following problem. The composition of the oxide semiconductor film varies in accordance with the film formation conditions or manufacturing conditions of the thin film transistor, and thus a defect may be caused to easily fluctuate the electric characteristics of the thin film transistor.

For example, Japanese Laid-Open Patent Publication No. 2010-153842 discloses a thin film transistor including a first oxide semiconductor region as an active region. The thin film transistor includes a second oxide semiconductor region formed between the first oxide semiconductor region and a protective insulating layer, and the second oxide semiconductor region has a conductivity lower than that of the first oxide semiconductor region and acts as a protective layer. With such a structure, the first oxide semiconductor region is prevented from being changed in the composition or from being deteriorated in the film quality, and thus the thin film transistor has stable electric characteristics.

SUMMARY

A transistor in an embodiment according to the present invention includes an oxide semiconductor layer on a substrate, the oxide semiconductor layer including a first region and a second region, a first gate electrode including a region overlapping the oxide semiconductor layer, the first gate electrode being arranged on a surface of the oxide semiconductor layer opposite to the substrate, a first insulating layer between the first gate electrode and the oxide semiconductor layer, and a first oxide conductive layer and a second oxide conductive layer between the oxide semiconductor layer and the substrate, the first oxide conductive layer and the second oxide conductive layer each including a region in contact with the oxide semiconductor layer. The first region and the second region include a region overlapping each other, the first region is arranged on the substrate side, and the second region is arranged on a side opposite to the substrate. A carrier concentration of the second region is lower than a carrier concentration of the first region.

A transistor in an embodiment according to the present invention includes an oxide semiconductor layer on a substrate, the oxide semiconductor layer having a first surface on a side of the substrate and a second surface opposite to the first surface, a first gate electrode including a region overlapping the oxide semiconductor layer, the first gate electrode being arranged on the second surface, a first insulating layer between the first gate electrode and the oxide semiconductor layer, and a first oxide conductive layer and a second oxide conductive layer between the oxide semiconductor layer and the substrate, the first oxide conductive layer and the second oxide conductive layer each including a region in contact with the first substrate. A carrier concentration of the second surface side is lower than a carrier concentration of the first surface side.

A method of manufacturing a transistor in an embodiment according to the present invention, the method includes, forming a first oxide conductive layer and a second oxide conductive layer on a substrate, forming an oxide semiconductor layer including a first region and a second region, the first region contacting with the first oxide conductive layer and the second oxide conductive layer, and the second region contacting with the first region on the side opposite to the substrate side, forming a first insulating layer covering the oxide semiconductor layer, and forming a first gate electrode on the first insulating layer, the first gate electrode having a region overlapping with the oxide semiconductor layer. A carrier concentration of the second region is lower than a carrier concentration of the first region.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing a structure of a transistor in an embodiment according to the present invention;

FIG. 2A is an energy band diagram of an oxide semiconductor layer in an embodiment according to the present invention;

FIG. 2B is an energy band diagram of an oxide semiconductor layer in an embodiment according to the present invention;

FIG. 3 is a diagram showing electrical characteristics of a transistor in an embodiment according to the present invention;

FIG. 4A shows a method for producing the transistor in an embodiment according to the present invention, and shows a stage of forming a second insulating layer, a second conductive film, and a third conductive film;

FIG. 4B shows the method for producing the transistor in an embodiment according to the present invention, and shows a stage of forming a photoresist film and exposing with a multi-gradation photomask;

FIG. 5A shows the method for producing the transistor in an embodiment according to the present invention, and shows a stage where a resist mask is formed;

FIG. 5B shows the method for producing the transistor in an embodiment according to the present invention, and shows a stage of etching a second conductive film and a third conductive film;

FIG. 6A shows the method for producing the transistor in an embodiment according to the present invention, and shows a stage of etching the third conductive film;

FIG. 6B shows the method for producing the transistor in an embodiment according to the present invention, and shows a stage of forming an oxide semiconductor layer;

FIG. 7A shows the method for producing the transistor in an embodiment according to the present invention, and shows a stage of forming a first insulating layer and a fourth conductive film;

FIG. 7B shows the method for producing the transistor in an embodiment according to the present invention, and shows a structure of a transistor;

FIG. 8 is a plan view showing a structure of a display device in an embodiment according to the present invention;

FIG. 9 shows an equivalent circuit of a pixel in the display device in an embodiment according to the present invention;

FIG. 10 is a plan view showing a structure of a pixel in the display device in an embodiment according to the present invention;

FIG. 11A is a cross-sectional view showing the structure of the pixel in the display device in an embodiment according to the present invention, taken along line A1-A2 in FIG. 10;

FIG. 11 B is a cross-sectional view showing the structure of the pixel in the display device in an embodiment according to the present invention, taken along line B1-B2 in FIG. 10;

FIG. 12A shows a structure of a bottom contact bottom gate type transistor and an influence of charges during an operation of the display device;

FIG. 12B shows a structure of a top contact bottom gate type transistor and an influence of charges during an operation of the display device;

FIG. 13 shows a cross-sectional structure of a transistor in an embodiment according to the present invention;

FIG. 14 is a plan view showing a method for producing the display device in an embodiment according to the present invention;

FIG. 15A is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line A1-A2 in FIG. 14;

FIG. 15B is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line B1-B2 in FIG. 14;

FIG. 16A is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line A1-A2 in FIG. 14;

FIG. 16B is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line B1-B2 in FIG. 14;

FIG. 17 is a plan view showing the method for producing the display device in an embodiment according to the present invention;

FIG. 18A is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line A1-A2 in FIG. 17;

FIG. 18B is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line B1-B2 in FIG. 17;

FIG. 19 is a plan view showing a method for producing the display device in an embodiment according to the present invention;

FIG. 20A is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line A1-A2 in FIG. 19;

FIG. 20B is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line B1-B2 in FIG. 19;

FIG. 21 is a plan view showing the method for producing the display device in an embodiment according to the present invention;

FIG. 22A is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line A1-A2 in FIG. 21;

FIG. 22B is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line B1-B2 in FIG. 21;

FIG. 23A is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line A1-A2 in FIG. 21;

FIG. 23B is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line B1-B2 in FIG. 21;

FIG. 24A is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line A1-A2 in FIG. 21;

FIG. 24B is a cross-sectional view showing the method for producing the display device in an embodiment according to the present invention, taken along line B1-B2 in FIG. 21;

FIG. 25A is a plan view of showing a photomask used for manufacturing a display device in an embodiment according to the present invention;

FIG. 25B is a plan view of showing a photomask used for manufacturing a display device in an embodiment according to the present invention;

FIG. 25C is a plan view of showing a photomask used for manufacturing a display device in an embodiment according to the present invention;

FIG. 26 is a cross-sectional view showing a structure of a transistor in an embodiment according to the present invention;

FIG. 27 shows an equivalent circuit of a pixel in the display device in an embodiment according to the present invention;

FIG. 28 is a plan view showing a structure of a pixel in the display device in an embodiment according to the present invention;

FIG. 29A is a cross-sectional view showing the structure of the pixel in the display device in an embodiment according to the present invention, taken along line A1-A2 in FIG. 28;

FIG. 29B is a cross-sectional view showing the structure of the pixel in the display device in an embodiment according to the present invention, taken along line B1-B2 in FIG. 28;

FIG. 30A is a cross-sectional view showing a structure of a transistor in an embodiment according to the present invention;

FIG. 30B is a cross-sectional view showing a structure of a transistor in an embodiment according to the present invention;

FIG. 31A is a plan view of showing a photomask used for manufacturing a display device in an embodiment according to the present invention;

FIG. 31B is a plan view of showing a photomask used for manufacturing a display device in an embodiment according to the present invention;

FIG. 32 is a cross-sectional view showing a structure of a transistor in an embodiment according to the present invention; and

FIG. 33 is a cross-sectional view showing a structure of a transistor in an embodiment according to the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described with reference to the drawings and the like. The present invention may be carried out in various embodiments, and should not be construed as being limited to any of the following embodiments. In the drawings, components may be shown schematically regarding the width, thickness, shape and the like, instead of being shown in accordance with the actual sizes, for the sake of clear illustration. The drawings are merely examples and do not limit the present invention in any way. In the specification and the drawings, components that are substantially the same as those described or shown previously bear the identical reference signs thereto (or the identical reference signs followed by letters “a”, “b” or the like), and detailed descriptions thereof may be omitted. The terms “first”, “second” and the like used for elements are merely provided for distinguishing the elements and do not have any other significance unless otherwise specified.

In the specification and the claims, an expression that a component is “on” another component encompasses a case where such a component is in contact with the another component and also a case where such a component is above or below the another component, namely, a case where still another component is provided between such a component and the another component, unless otherwise specified.

An oxide semiconductor is a type of compound semiconductor containing a metal material and oxygen. Therefore, in a process of manufacturing a thin film transistor, it is required to control the composition and the oxygen deficiency of the oxide semiconductor, and also to control the impurities. A thin film transistor of a bottom-gate type described in Japanese Laid-Open Patent Publication No. 2010-153842 has the following problems when being included in a liquid crystal display device or an organic EL display device. In the liquid crystal display device, a back channel-side potential is easily influenced by a common potential. In the case where an organic EL element of an inverted stack structure is provided in a pixel of the organic EL display device, a back channel-side potential is easily influenced by the positive potential of an anode electrode, which is significantly serious. In addition, in the case where the source electrode and the drain electrode are formed of copper (Cu), the copper (Cu) atoms have a strong action as carrier (electron) killers against an n-type oxide semiconductor. In the structure described in Japanese Laid-Open Patent Publication No. 2010-153842, there is a problem that the copper (Cu) atoms contaminate the channel region. In order to control the characteristics of the thin film transistor by the device structure, provision of a back gate, for example, is considered to be effective. However, a thin film transistor including a back gate has problems of being complicated in the structure and of requiring a larger number of photomasks to be manufactured. In some embodiments described below, a transistor and a display device capable of overcoming one or a plurality of these problems will be described.

Embodiment 1 1-1. Transistor Structure

FIG. 1 is a cross-sectional view showing a structure of a transistor 100 a in embodiment 1 according to the present invention. The transistor 100 a includes a second insulating layer 106, an oxide semiconductor layer 112, a first insulating layer 114, and a first gate electrode 116, which are provided on a substrate 102 having an insulating surface.

The first gate electrode 116 is located to face one surface of the oxide semiconductor layer 112 (surface opposite to a surface facing the substrate 102). The first insulating layer 114 is located between the oxide semiconductor layer 112 and the first gate electrode 116. The first gate electrode 116 and the oxide semiconductor layer 112 are located to partially overlap each other while having the first insulating layer 114 between the first gate electrode 116 and the oxide semiconductor layer 112. In the transistor 100 a, a channel is formed in a region where the oxide semiconductor layer 112 overlaps the first gate electrode 116. The first insulating layer 114 acts as a gate insulating film in a region where the oxide semiconductor layer 112 and the first gate electrode 116 overlap each other.

The second insulating layer 106 is located between the oxide semiconductor layer 112 and the substrate 102. A first oxide conductive layer 108 a and a second oxide conductive layer 108 b are located between the oxide semiconductor layer 112 and the second insulating layer 106. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are provided in contact with the oxide semiconductor layer 112. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located such that one end part of the first oxide conductive layer 108 a and one end part of the second oxide conductive layer 108 b overlap the first gate electrode 116. One of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b acts as a source region, and the other of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b acts as a drain region. In the structure shown in FIG. 1, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located such that one end part of the first oxide conductive layer 108 a and one end part of the second oxide conductive layer 108 b overlap the first gate electrode 116. Therefore, the oxide semiconductor layer 112 does not include any offset region (region having a high resistance). Thus, the level of on-current is increased.

A first line 110 a is provided in contact with the first oxide conductive layer 108 a, and a second line 110 b is provided in contact with the second oxide conductive layer 108 b. The first line 110 a is located between the first oxide conductive layer 108 a and the oxide semiconductor layer 112, and the second line 110 b is located between the second oxide conductive layer 108 b and the oxide semiconductor layer 112. The first line 110 a and the second line 110 b are respectively located in contact with the first oxide conductive layer 108 a and the second oxide conductive layer 108 b. This decreases the number of photolithography steps as described below.

1-2. Oxide Semiconductor Layer

The oxide semiconductor layer 112 is formed of a transparent oxide semiconductor containing one or a plurality of elements selected from indium (In), zinc (Zn), gallium (Ga), tin (Sn), aluminum (Al), tungsten (W) and silicon (Si). For example, an oxide semiconductor material used to form the oxide semiconductor layer 112 may be one of a four-component oxide material, a three-component oxide material, and a two-component oxide material showing semiconductor characteristics. Examples of the four-component oxide material include an In₂O₃-Ga₂O₃-SnO₂-ZnO-based oxide material, and the like. Examples of the three-component oxide material include an In₂O₃-Ga₂O₃-SnO₂-based oxide material, an In₂O₃-Ga₂O₃-ZnO-based oxide material, an In₂O₃-SnO₂-ZnO-based oxide material, an In₂O₃-Al₂O₃-ZnO-based oxide material, a Ga₂O₃-SnO₂-ZnO-based oxide material, a Ga₂O₃-Al₂O₃-ZnO-based oxide material, an SnO₂-Al₂O₃—ZnO-based oxide material, and the like. Examples of the two-component oxide material include an In₂O₃-SnO₂-based oxide material, an In₂O₃-ZnO-based oxide material, an SnO₂-ZnO-based oxide material, an Al₂O₃—ZnO-based oxide material, a Ga₂O₃-ZnO-based oxide material, an SnO₂-SiO₂-based oxide material, an In₂O₃-W₂O₃-based oxide material, and the like. It is especially preferred to use an In₂O₃-Ga₂O₃-SnO₂-based oxide material. The above-listed oxide semiconductors may each contain tantalum (Ta), scandium (Sc), nickel (Ni), lanthanum (La), magnesium (Mg), hafnium (Hf), yttrium (Y) or titanium (Ti). The In-Ga-Sn-O-based oxide materials listed above are each an oxide material containing at least In, Ga and Sn. There is no specific limitation on the composition ratio thereof. Regarding the composition ratio, it is preferred that each of the In-Ga-Sn-O-based oxide materials contains In at 60 atm % to 70 atm %, contains Ga at 10 atm % to 25 atm %, and contains Sn at 5 atm % to 30 atm % with respect to the total of In, Ga and Sn. In other words, the oxide semiconductor layer 112 may be formed of a thin film represented by chemical formula InMO₃(ZnO)_(m) (m>0). M represents one or a plurality of metal elements selected from Sn, Ga, Zn, Sc, La, Y, Ni, Al, Mg, Ti, Ta, W, Hf and Si. The oxide material contained in each of the four-component oxide materials, the three-component oxide materials and the two-component oxide materials listed above is not limited to having a stoichiometric composition, but may have a composition shifted from the stoichiometric composition.

The oxide semiconductor layer 112 includes a first region 112-1 and a second region 112-2 stacked from the substrate 102 side. The first region 112-1 is located at a first surface, of the oxide semiconductor layer 112, facing the substrate 102, and the second region 112-2 is located at a second surface opposite to the first surface. In the oxide semiconductor layer 112, the first region 112-1 is thicker than the second region 112-2. It is preferred that the first region 112-1 of the oxide semiconductor layer 112 has a thickness of 30 nm to 100 nm. It is preferred that the second region 112-2 of the oxide semiconductor layer 112 has a thickness of 2 nm to 10 nm. The thickness of the first region 112-1 or the second region 112-2 is not limited to such a range. It is sufficient that the oxide semiconductor layer 112 including the first region 112-1 and the second region 112-2 has a thickness of 20 nm to 100 nm, for example, 30 nm to 60 nm.

The first region 112-1 and the second region 112-2 of the oxide semiconductor layer 112 are different from each other in the carrier concentration (concentration of the carrier present in a larger amount). The carrier concentration of the second region 112-2 is of a smaller value than that of the carrier concentration of the first region 112-1. It is preferred that the carrier concentration of the first region 112-1 is about 1×10¹⁵/cm³ to about 5×10¹⁸/cm³. It is preferred that the carrier concentration of the second region 112-2 is about 1×10¹¹/cm³ to about 1×10¹⁵/cm³. In correspondence with this, it is preferred that the first region 112-1 of the oxide semiconductor layer 112 has a conductivity of about 1×10⁻⁵ S/cm to about 10 S/cm, and that the second region 112-2 of the oxide semiconductor layer 112 has a conductivity of about 1×10⁻¹⁰ S/cm to about 1×10⁻⁵ S/cm. It is also preferred that the second region 112-2 of the oxide semiconductor layer 112 has a carrier mobility smaller than a carrier mobility of the first region 112-1 of the oxide semiconductor layer 112.

The first region 112-1 and the second region 112-2 of the oxide semiconductor layer 112 may be different from each other in the crystallinity. It is preferred that the second region 112-2 of the oxide semiconductor layer 112 has a crystallization ratio higher than a crystallization ratio of the first region 112-1. The first region 112-1 of the oxide semiconductor layer 112 may be in a state of an amorphous phase, or a mixed state of an amorphous phase and a nano-crystalline phase. The second region 112-2 of the oxide semiconductor layer 112 may be in a state of a nano-crystalline phase or a mixed state of an amorphous phase and a nano-crystalline phase. In the case of being such a mixed state, the second region 112-2 may have a ratio of the nano-crystalline phase higher than that of the first region 112-1, and may further contain a polycrystalline phase.

The oxide semiconductor layer 112 may be formed by sputtering. The first region 112-1 and the second region 112-2 may be formed with different sputtering conditions. For example, the first region 112-1 of the oxide semiconductor layer 112 is formed by use of noble gas such as Ar or the like as sputtering gas, whereas the second region 112-2 is formed by use of noble gas such as Ar or the like and oxygen gas as sputtering gas. The partial pressure of oxygen may be made higher for forming the second region 112-2 than for forming the first region 112-1, so that the second region 112-2 has less donor defects and thus has an increased crystallization ratio. As a result, the carrier concentration of the second region 112-2 is made lower than that of the first region 112-1, and in correspondence with this, the conductivity of the second region 112-2 is made lower than that of the first region 112-1. Oxygen may be incorporated into the sputtering gas for forming the second region 112-2 (the partial pressure of oxygen is increased), so that the second region 112-2 is formed of a finer film (denser film) than that of the first region 112-1.

The first region 112-1 and the second region 112-2 of the oxide semiconductor layer 112 may have the same composition but have different crystallization ratios from each other. Alternatively, the first region 112-1 and the second region 112-2 of the oxide semiconductor layer 112 may be formed of the same type of metal oxide material but have different compositions from each other. Still alternatively, the first region 112-1 and the second region 112-2 of the oxide semiconductor layer 112 may be formed of metal oxide materials having different compositions from each other. The first region 112-1 and the second region 112-2 may be formed in any of the above-described manners, so as to be different from each other in the carrier concentration and the conductivity.

FIG. 2A shows an example of band diagram of the oxide semiconductor layer 112 in the case where the first region 112-1 and the second region 112-2 are different from each other in the crystallization ratio. For example, the oxide semiconductor layer 112 may be formed of an In₂O₃-Ga₂O₃-ZnO-based oxide material, and the first region 112-1 may be in an amorphous state or in a mixed state of an amorphous phase and a nano-crystalline phase. The second region 112-2 may be in a nano-crystalline phase or a mixed state of an amorphous phase and a nano-crystalline phase. It is preferred that the ratio of the nano-crystalline component is higher in the second region 112-2 than in the first region 112-1.

Although the oxide semiconductor layer 112 has a single composition, the first region 112-1 and the second region 112-2 are different from each other in the crystallization ratio. Therefore, the first region 112-1 and the second region 112-2 are different from each other in the bandgap. The bandgap of the first region 112-1 is 2.8 eV to 3.0 eV, whereas the bandgap of the second region 112-2 is 3.0 eV to 3.2 eV. The bandgap of the first region 112-1 is of a smaller value than that of the bandgap of the second region 112-2. In correspondence with the difference in the crystallization ratio, the first region 112-1 has a work function larger than a work function of the second region 112-2. Therefore, in the band diagram of the first region 112-1 and the second region 112-2 in a joined state, the second region 112-2 has an energy (Ec) higher than that of the first region 112-1 at the bottom of the conduction band.

In the case where the oxide semiconductor layer 112 having such a structure is included in the transistor 100 a, the second region 112-2 is provided, between the first insulating layer 114 and the first region 112-1, to form an energy barrier to the electrons in the conduction band. With such a structure, the transistor 100 a includes a channel region separated from an interface between the first insulating layer 114 and the oxide semiconductor layer 112. The channel is buried in the oxide semiconductor layer 112 and is operable, so that the transistor 100 a prevents the carriers (electrons) from being trapped at the interface between the first insulating layer 114 and the oxide semiconductor layer 112.

FIG. 2B shows an example of band diagram of the oxide semiconductor layer 112 in the case where the first region 112-1 and the second region 112-2 are formed of oxide materials having different compositions from each other. For example, the first region 112-1 may be formed of an In₂O₃-Ga₂O₃-SnO₂-ZnO-based oxide material, an In₂O₃-Ga₂O₃-SnO₂-based oxide material or an In₂O₃-Ga₂O₃-ZnO-based oxide material. The second region 112-2 may be formed of a gallium oxide material such as a Ga₂O₃-based oxide material, a GaSnO_(x)-based oxide material, a GaSiO_(x)-based oxide material or the like. The gallium oxide material is a wide gap material, and may form a bandgap of 4 eV or larger. For example, the bandgap of the In₂O₃-Ga₂O₃-SnO₂-based oxide material usable to form the first region 112-1 is 2.8 eV to 3.0 eV. The oxide materials usable to form the second region 112-2 have the following bandgaps. The bandgap of a-Ga₂O₃ is 4.3 eV. The bandgap of a-GaSnO_(x) is 4.0 eV. The bandgap of a-GaSiO_(x) is 4.5 eV or larger. Thus, the bandgap of the second region 112-2 may be larger than that of the first region 112-1 by at least 1.0 eV.

The oxide material used to form the first region 112-1 may further contain 2 atm % to 5 atm % of silicon (Si). Silicon contained in the oxide material used to form the first region 112-1 improves the field effect mobility and the heat resistance of the transistor 100 a. As a result, the threshold voltage is made controllable.

As shown in FIG. 2B, the first region 112-1 and the second region 112-2 formed of oxide semiconductor materials of different compositions are different from each other in the bandgap. The bandgap of the first region 112-1 is smaller than that of the second region 112-2, and the work function of the first region 112-1 is larger than that of the second region 112-2. Therefore, like in FIG. 2A, in the band diagram of the first region 112-1 and the second region 112-2 in a joined state, the second region 112-2 has an energy (Ec) higher than that of the first region 112-1 at the bottom of the conduction band. With such a structure, the transistor 100 a includes a channel region separated from an interface between the first insulating layer 114 and the oxide semiconductor layer 112. In other words, the transistor 100 a prevents the carriers (electrons) from being trapped at the interface between the first insulating layer 114 and the oxide semiconductor layer 112.

In the case where the first region 112-1 and the second region 112-2 are formed of oxide semiconductor layers containing different elements from each other, it is sufficient that the bandgap of the Ga oxide material used to form the second region 112-2 is larger by at least 1 eV than the bandgap of the oxide material used to form the first region 112-1. For example, the first region 112-1 of the oxide semiconductor layer 112 may be formed of an In₂O₃-Ga₂O₃-ZnO-based oxide material, whereas the second region 112-2 may be formed of a Ga₂O₃-based oxide material.

In the case where an oxide semiconductor layer is formed by sputtering, the ion sheath is extinguished at the stage of stopping glow discharge. After this, sputtered particles remaining in the gas phase are deposited to form a low-density region. This low-density region is considered to influence the transistor characteristics. In the transistor 100 a in this embodiment, the oxide semiconductor layer 112 includes the first region 112-1 and the second region 112-2 to overcome this problem.

In the oxide semiconductor layer 112 of the transistor 110 a, the second region 112-2 is lower in the conductivity, and thus in the carrier concentration, than the first region 112-1, so that the carriers do not easily flow at the interface between the first insulating layer 114 and the oxide semiconductor layer 112. The second region 112-2 is higher in the crystallization ratio than the first region 112-1, so that a finer film is formed in the second region 112-2. The second region 112-2 is wider in the bandgap of the energy band than the first region 112-1, so that a channel region is formed in the oxide semiconductor layer 112 (namely, a buried channel-type transistor is formed).

With such a structure, the transistor 100 a decreases the level of charges trapped at the interface between the first insulating layer 114 and the oxide semiconductor layer 112 and prevents the threshold voltage from being shifted. Thus, the transistor 100 a is realized as a normally-off transistor. Since the transistor 100 a is of a buried channel-type, the leak current is suppressed from flowing at the interface between the first insulating layer 114 and the oxide semiconductor layer 112, and thus the level of off-current is decreased. As shown in FIG. 1, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b acting as the source region and the drain region are in contact with the first region 112-1, which has a high conductivity. Therefore, the level of on-current is increased. As a result, the transistor 100 a obtains a ratio of an on-current and an off current (on/off ratio) of about 1×10⁹ to about 1×10¹².

An intermediate region, formed of an oxide semiconductor, that has a carrier concentration changing step by step or continuously may be present between the first region 112-1 and the second region 112-2. The intermediate region of the oxide semiconductor may be formed in the oxide semiconductor layer in which the first region 112-1 and the second region 112-2 are formed, or may be separately formed in a different oxide semiconductor layer.

1-3. Oxide Conductive Layer

The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are formed of a metal oxide material, a metal nitride material, or a metal oxide nitride material, all of which are conductive. Examples of the metal oxide material usable for the first oxide conductive layer 108 a and the second oxide conductive layer 108 b include indium tin oxide (In₂O₃.SnO₂: ITO), indium tin zinc oxide (In₂O₃.SnO₂.ZnO: ITZO), indium tin silicon oxide (In₂O₃.SnO₂.SiO₂: ITSO), tin oxide (SnO₂), aluminum zinc tin oxide (Al₂O₃.ZnO.SnO₂: AZTO), gallium zinc tin oxide (Ga₂O₃.ZnO.SnO₂: GZTO), zinc tin oxide (ZnO.SnO₂: ZTO), gallium tin oxide (Ga₂O₃.SnO₂: GTO), and the like. Such a metal oxide material forms a good ohmic contact with the first region 112-1 of the oxide semiconductor layer 112.

Examples of the metal oxide material usable for the first oxide conductive layer 108 a and the second oxide conductive layer 108 b also include titanium oxide (TiO_(x)) and the like. Examples of the metal nitride material usable for the first oxide conductive layer 108 a and the second oxide conductive layer 108 b include titanium nitride (TiN_(x)), zirconium nitride (ZrN_(x)), and the like. Examples of the metal oxide nitride material usable for the first oxide conductive layer 108 a and the second oxide conductive layer 108 b include titanium oxide nitride (TiO_(x)N_(y)), tantalum oxide nitride (TaO_(x)N_(y)), zirconium oxide nitride (ZrO_(x)N_(y)), hafnium oxide nitride (HfO_(x)N_(y)), and the like. The metal oxide materials, the metal nitride materials, and the metal oxide nitride materials described above may contain trace amount of metal element in order to improve the conductivity. For example, titanium oxide doped with niobium (TiO_(x):Nb) may be used. A high melting point metal silicide oxide such as TiSiO_(x) or the like may be used. Use of such a metal oxide material, such a metal nitride material, or such a metal oxide nitride material that exhibits an n-type electric conductivity guarantees stability even in the case where the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are in contact with the first line 110 a and the second line 110 b, respectively. Namely, use of such a metal oxide material, such a metal nitride material, or such a metal oxide nitride material prevents an oxidation-reduction reaction (local cell reaction) with aluminum (Al) having a lower potential.

1-4. Insulating Layers

The second insulating layer 106 and the first insulating layer 114 are formed of an inorganic insulating material. Examples of the inorganic insulating material usable for the second insulating layer 106 and the first insulating layer 114 include silicon oxide, silicon nitride, silicon oxide nitride, aluminum oxide, and the like. The second insulating layer 106 and the first insulating layer 114 each have a single-layer structure, or a stack structure including a plurality of films, formed of such an organic insulating material. For example, the second insulating layer 106 may include a silicon nitride film and a silicon oxide film stacked in this order from the substrate 102 side. The first insulating layer 114 may include a silicon oxide film and a silicon nitride film stacked in this order from the oxide semiconductor layer 112 side. The second insulating layer 106 and the first insulating layer 114, in the case of including a plurality of organic insulating films, alleviate the action of an internal stress and also improve the barrier property against water vapor or the like.

It is preferred that surfaces of the second insulating layer 106 and the first insulating layer 114 that are in contact with the oxide semiconductor layer 112 are formed of a silicon oxide film, a silicon oxide nitride film or an aluminum oxide film. Since such an insulating layer containing an oxide inorganic insulating material is in contact with the oxide semiconductor layer 112 (in other words, an insulating layer containing a nitride inorganic insulating material is not in contact with the oxide semiconductor layer 112), diffusion of impurities such as hydrogen or the like, which generates a donor in the oxide semiconductor layer 112, is suppressed. Since the oxide insulating film is provided in contact with the oxide semiconductor layer 112, a defect (donor) caused by oxygen deficiency is prevented from being caused to the oxide semiconductor layer 112.

A silicon-based material of the silicon oxide film that is included in the second insulating layer 106 and the first insulating layer 114 and is in contact with the oxide semiconductor layer 112 may be represented by Si_(α)X_(β). In this formula, it is preferred that X contains at least one of a cyanate group, i.e., —O—CN, an isocyanate group, i.e., —N═C═O, a cyano group, i.e., a diazo group, i.e., ═N₂, an azide group, i.e., —N₃, a nitroso group, i.e., —NO, and a nitro group, i.e., —NO₂. It is preferred that α is 1 to 3, and that β is 1 to 8. For example, it is preferred to use any one of tetracyanatesilane represented by structural formula (1), tetraisocyanatesilane represented by structural formula (2), tetracyanosilane represented by structural formula (3), 1,1,1,1-isocyanatecyanatecyanonitrosilane represented by structural formula (4), 1,1-diisocyanate 1,1-dicyanatesilane represented by structural formula (5), hexaisocyantesilane represented by structural formula (6) and octaisocyanatesilane represented by structural formula (7). The structural formulas are shown below. In the formation of a silicon oxide film, it is preferred to use, as oxygen gas, at least one of O₂, O₃, NO, NO₂, N₂O, N₂O₃, N₂O₄, N₂O₅, CO and CO₂.

As described above, a hydrogen-free silicon-based material may be used for the silicon oxide film that is included in the second insulating layer 106 and the first insulating layer 114 and is in contact with the oxide semiconductor layer 112, so that the diffusion of impurities such as hydrogen or the like that generates a donor in the oxide semiconductor layer 112 is more suppressed.

For example, the silicon oxide film that is included in the second insulating layer 106 and the first insulating layer 114 and is in contact with the oxide semiconductor layer 112 may be formed of a tetraisocyanatesilate-based material by use of oxygen-containing gas such as oxygen (O₂) gas, nitrous oxide (N₂O) gas or the like. In the case where a parallel flat plate-type plasma CVD device is used to form the silicon oxide film, it is preferred that the film formation is performed at a reaction gas pressure of 13 Pa or higher and 666 Pa or lower and a substrate temperature of 200° C. to 350° C. In the case where an ICP-DVD device is used, it is preferred that the film formation is performed at a reaction gas pressure of 1.3 Pa or higher and 66 Pa or lower and a substrate temperature of 200° C. to 300° C.

It is preferred that the second insulating layer 106 and the first insulating layer 114 each further include a silicon nitride film stacked on the silicon oxide film on the side opposite to the oxide semiconductor layer 112. It is preferred that a silicon-based material of the silicon nitride film is silicon tetrafluoride (SiF₄) gas. In the formation of the silicon nitride film, it is preferred to use nitrogen (N₂) as a supply source of nitrogen atoms.

As described above, a hydrogen-free silicon-based material may be used for the silicon nitride film that is included in the second insulating layer 106 and the first insulating layer 114 and is in contact with the silicon oxide film, so that an adverse influence of hydrogen by which the oxide semiconductor layer 112 is reduced to be conductive or that oxygen deficiency is caused to increase the carrier concentration is suppressed. For example, as the silicon nitride film that is included in the second insulating layer 106 and the first insulating layer 114 and is in contact with the silicon oxide film, a fluorine-containing silicon nitride (P—SiN_(x):F) film may be formed by use of an inductively coupled plasma chemical vapor deposition (ICP-CVD) device and by use of SiF₄ gas and N₂ gas.

The second insulating layer 106 and the first insulating layer 114 formed of hydrogen-free materials as described above may have a hydrogen content suppressed to about 10¹⁸/cm³ to about 10²⁰/cm³. In the transistor 100 a in this embodiment, the second insulating layer 106 and the first insulating layer 114 may each adopt a structure in which a silicon oxide film formed of a tetraisocyanatesilane-based material, and a fluorine-containing silicon nitride film (SiN_(x): F) formed by use of SiF₄ as a material, are stacked on each other. With such a structure, the carrier concentration of the oxide semiconductor layer 112 is suppressed from being increased, and thus the threshold voltage of the transistor 100 a is suppressed from being fluctuated.

1-5. Gate Electrodes

The first gate electrode 116 is formed of a metal material such as aluminum (Al), molybdenum (Mo), tungsten (W), zirconium (Zr) or the like. For example, the first gate electrode 116 may be formed of a film of aluminum (Al), a molybdenum-tungsten alloy (MoW), or the like. The first gate electrode 116 may be formed of an aluminum alloy, a copper alloy, or a silver alloy. Examples of the aluminum alloy usable for the first gate electrode 116 include an aluminum-neodymium alloy (Al—Nd), an aluminum-neodymium-nickel alloy (Al—Nd—Ni), an aluminum-carbon-nickel alloy (Al—C—Ni), a copper-nickel alloy (Cu—Ni), and the like. Alternatively, the first gate electrode 116 may be formed of a transparent conductive film of indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO) or the like. Still alternatively, an electrode having a stack structure of three layers of Mo/Al/Mo, Mo/Cu/Mo or the like is effective. Namely, a three-layer stack structure in which any of the above-listed metal materials is sandwiched by oxidation-preventive layers containing molybdenum (Mo), zirconium (Zr), titanium (Ti) or an alloy thereof may be used.

1-6. Lines

The first line 110 a and the second line 110 b are formed of a metal material having a high conductivity such as aluminum (Al), copper (Cu) or the like. For example, the first line 110 a and the second line 110 b are formed of an aluminum alloy, a copper alloy, or a silver alloy. Examples of the aluminum alloy usable for the first line 110 a and the second line 110 b include an aluminum-neodymium alloy (Al—Nd), an aluminum-titanium alloy (Al—Ti), an aluminum-silicon alloy (Al—Si), an aluminum-neodymium-nickel alloy (Al—Nd—Ni), an aluminum-carbon-nickel alloy (Al—C—Ni), a copper-nickel alloy (Cu—Ni), and the like. Use of such a metal material provides heat resistance and decreases the line resistance. Alternatively, a line having a stack structure of three layers of Mo/Al/Mo, Mo/Cu/Mo or the like is effective. Namely, a three-layer stack structure in which any of the above-listed metal materials is sandwiched by oxidation-preventive layers containing molybdenum (Mo), zirconium (Zr), titanium (Ti) or an alloy thereof may be used.

1-7. Operation and Function of the Transistor

The transistor 100 a includes the first gate electrode 116 provided on one surface of the oxide semiconductor 112 (surface opposite to the surface facing the substrate 102). In a region where the oxide semiconductor layer 112 overlaps the first gate electrode 116, the channel is formed. The second region 112-2 of the oxide semiconductor layer 112 has a carrier concentration lower than that of the first region 112-1. Therefore, the transistor 100 a includes the channel formed in the first region 112-1 in the oxide semiconductor layer 112. The electric current flows in an area, in the first region 112-1 of the oxide semiconductor layer 112, that is closer to the second region 112-2 (on the first gate electrode 116 side), so that the field effect mobility of the transistor 100 a is improved.

In the transistor 100 a in this embodiment, the second region 112-2 of the oxide semiconductor layer 112 is formed by use of mixed gas of noble gas and oxygen (O₂). The partial pressure of oxygen used to form the second region 112-2 of the oxide semiconductor layer 112 is higher than the partial pressure of oxygen used to form the first region 112-1. FIG. 3 shows an electric characteristic (Vg-Id characteristic) of a transistor when the partial pressure of oxygen used to form the second region 112-2 of the oxide semiconductor layer 112 is changed. As shown in FIG. 3, the partial pressure of oxygen used to form the second region 112-2 of the oxide semiconductor layer 112 may be changed to 5%, 10%, 15%, 20% and 50%, so that the threshold voltage of the transistor is adjusted. For the transistor 100 a in this embodiment, the threshold voltage is preferably in the range of 0 V to 5 V, and is more preferably in the range of 0.5 V to 3 V. The first insulating layer 114 may be formed with no use of reducing gas, so that the substrate temperature is allowed to be raised to about 250° C. to about 350° C. at the time of film formation. This makes it unnecessary to perform annealing at a high temperature for a long time after the formation of the transistor 100 a is completed. Thus, the reliability is improved. In the case where the second region 112-2 of the oxide semiconductor layer 112 is formed at a partial pressure of oxygen in the range of 15% to 20%, the change in the threshold voltage, caused due to a stress such as heat or the like, is decreased. Therefore, even if annealing is performed several times at 220° C. to 250° C. in order to form a color filter after the formation of the transistor 100 a is completed, the transistor characteristics are maintained stably.

The transistor 100 a includes the first oxide conductive layer 108 a and the second oxide conductive layer 108 b located on one surface of the oxide semiconductor layer 112 (surface facing the substrate 102). Therefore, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are in contact with the first region 112-1 of the oxide semiconductor layer 112. The first region 112-1 of the oxide semiconductor layer 112 has a conductivity higher than that of the second region 112-2. Therefore, the contact resistance between the first oxide conductive layer 108 a and the first region 112-1 of the oxide semiconductor layer 112, and the contact resistance between the second oxide conductive layer 108 b and the first region 112-1 of the oxide semiconductor layer 112, are decreased. In other words, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are in contact with the surface, of the oxide semiconductor layer 112, that faces the substrate 102, so that the contact resistance is decreased.

1-8. Manufacturing Method

Now, a method of manufacturing the transistor 100 a will be described. FIG. 4A shows a stage of forming the second insulating layer 106, a second conductive film 107, and a third conductive film 109 on a top surface of the substrate 102. From the second conductive film 107, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are formed. From the third conductive film 109, the first line 110 a and the second line 110 b are formed.

The substrate 102 is, for example, a transparent insulating substrate. The transparent insulating substrate is formed of non-alkali glass such as aluminosilicate glass, aluminoborosilicate glass or the like, or quartz. Among various types of transparent insulating substrate, a flexible transparent insulating substrate is formed of, for example, transparent polyimide, transparent polyamide or the like.

The second insulating layer 106 is formed of an inorganic insulating film. For example, the second insulating layer 106 may be a film formed of one or a plurality of materials selected from silicon oxide, silicon nitride, and silicon oxide nitride by plasma CVD (Chemical Vapor Deposition). Alternatively, the second insulating layer 106 may be an aluminum oxide film. In this case, the second insulating layer 106 is formed by sputtering by use of an alumina sputtering target.

The second conductive film 107 used to form the first oxide conductive layer 108 a and the second oxide conductive layer 108 b is formed of a metal oxide material, a metal nitride material, a metal oxide nitride material, or a high melting point metal silicide oxide material, all of which are conductive. The second conductive film 107 is formed by sputtering. The second conductive film 107 used to form the first oxide conductive layer 108 a and the second oxide conductive layer 108 b is formed of, for example, a conductive metal oxide material to have a thickness of 30 nm to 200 nm. The third conductive film 109 used to form the first line 110 a and the second line 110 b is formed of a metal material or an alloy material by sputtering. The third conductive film 109 used to form the first line 110 a and the second line 110 b is formed of a metal material to have a thickness of 200 nm to 2000 nm in order to have a low resistance.

FIG. 4B shows a lithography step of forming the first line 110 a, the second line 110 b, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b. In this example, a multi-gradation exposure method (halftone exposure method) is used. Specifically, the patterns of the first line 110 a, the second line 110 b, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are formed by one photomask.

A positive photoresist film 205 is formed on the third conductive film 109. A multi-gradation photomask 201 is used for exposing the photoresist film 205 to light. A multi-gradation photomask is available in two types: a gray tone photomask, which has a multi-gradation scale pattern having slits of a resolution equal to, or lower than, the resolution of an exposure device and realizes multi-gradation exposure by the slits blocking a part of light; and a halftone photomask, which realizes multi-gradation exposure by use of a semi-transmissive film. In this embodiment, both types of multi-gradation photomask are usable as the multi-gradation photomask 201. The photoresist film 205 is exposed to light through a transmissive region, a semi-transmissive region 202 and a non-transmissive region 203 of the multi-gradation photomask 201. As a result, three types of portions, specifically, an exposed portion, a gradation-exposed portion, and a non-exposed portion are formed in the photoresist film 205.

Then, the photoresist film 205 is developed to form a resist mask 207 a including regions having different thicknesses as shown in FIG. 5A. As shown in FIG. 5A, the resist mask 207 a is thicker in a region corresponding to regions of the third conductive film 109 where the first line 110 a and the second line 110 b are to be formed, and is thinner in a region corresponding to regions of the second conductive film 107 where the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are to be formed.

The third conductive film 109 and the second conductive film 107 are etched by use of the resist mask 207 a. There is no specific limitation on the conditions for the etching. For example, the third conductive film 109, which is formed of a metal material, is wet-etched by use of a mixed acid etchant, and the second conductive film 107, which is formed of a metal oxide material or the like, is dry-etched by use of chlorine-based gas or wet-etched by use of an oxalic acid-based material. On this stage, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are formed. After the etching, an ashing process is performed to remove the thinner region of the resist mask 207 a to expose a surface of the third conductive film 109. FIG. 5B shows a resist mask 207 b after the ashing process. The resist mask 207 b is left on the third conductive film 109.

Next, etching is performed on the exposed regions of the third conductive film 109. This etching is wet-etching performed by use of, for example, a mixed acid etchant. The second conductive film 107, which is formed of a metal oxide material or the like, is not easily etched away by the mixed acid etchant as long as containing tin (Sn) at a content of at least 10 atm %, and thus the selection ratio is relatively high. Therefore, the shape of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b below the third conductive film 109 is kept unchanged. FIG. 6A shows a stage where the first line 110 a and the second line 110 b are formed as a result of the etching performed on the third conductive film 109. After the third conductive film 109 is etched, the resist mask 207 b is removed by a resist releasing solution or ashing.

Surfaces of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b already formed are exposed to oxygen plasma by the resist releasing solution or the ashing process. However, titanium (Ti), tantalum (Ta), hafnium (Hf) or zirconium (Zr) contained as a component of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b does not generate any defect, that traps the carriers (electrons), even when being oxidized and becomes an n-type oxide semiconductor without expressing the role of a carrier (electron) killer. Therefore, even though being exposed to oxygen plasma, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b form a good contact with the oxide semiconductor layer 112 formed in a later step.

FIG. 6B show a stage of forming the oxide semiconductor layer 112. The oxide semiconductor layer 112 is formed on substantially the entirety of the transistor 100 a to cover the first oxide conductive layer 108 a, the second oxide conductive layer 108 b, the first line 110 a, and the second line 110 b. The oxide semiconductor layer 112 is formed by sputtering. As a sputtering target, a sintered oxide semiconductor material is used. The oxide semiconductor layer 112 is formed to have a thickness of 20 nm to 100 nm, for example, 30 nm to 60 nm.

The oxide semiconductor layer 112 may be formed by use of, for example, a sputtering target compatible to any of the four-component oxide materials, the three-component oxide materials, the two-component oxide materials and the one-component oxide materials. The first region 112-1 of the oxide semiconductor layer 112 may be formed by use of, as sputtering gas, noble gas such as argon (Ar), xenon (Xe) or the like. The second region 112-2 of the oxide semiconductor layer 112 may be formed by use of mixed gas of noble gas and oxygen (O₂). The partial pressure of oxygen used to form the second region 112-2 of the oxide semiconductor layer 112 is adjusted to be higher than the partial pressure of oxygen used to form the first region 112-1. It is preferred that the partial pressure of oxygen used to form the second region 112-2 of the oxide semiconductor layer 112 is, for example, 10% to 50%. It is more preferred that the partial pressure of oxygen used to form the second region 112-2 of the oxide semiconductor layer 112 is 15% to 30%. The second region 112-2 may be formed by use of mixed gas containing noble gas (Ar gas), oxygen (O₂) and nitrogen (N2) or nitrous oxide (N₂O). Specifically, at the stage of finishing the film formation of the first region 112-1, oxygen gas, in addition to noble bas (Ar gas), is introduced into the film formation chamber while the glow discharge is maintained. At this point, the air discharge rate may be increased to decrease the pressure in the film formation chamber and thus to decrease the film formation rate. With such a control, the second region 112-2 can be form with a higher density.

The first region 112-1 and the second region 112-2 of the oxide semiconductor layer 112 in this embodiment may be formed continuously by controlling the partial pressure of oxygen in this manner, so that the productivity thereof is improved. The partial pressure of oxygen used to form the second region 112-2 of the oxide semiconductor layer 112 is increased to increase the crystallization ratio of the film of the second region 112-2. Thus, the second region 112-2 is not easily reduced even if being exposed to plasma during the formation of the first insulating layer 114 by plasma CVD. This allows the substrate temperature during the formation of the first insulating layer 114 to be raised to 250° C. or higher, and thus the resultant SiO₂ film is of a high quality. This improves the reliability of the transistor 100 a.

In one example, the first region 112-1 and the second region 112-2 of the oxide semiconductor layer 112 in this embodiment are formed continuously by using the same sputtering target and controlling the partial pressure of oxygen of the sputtering gas. The method of forming the first region 112-1 and the second region 112-2 is not limited to this. For example, the entirety of the oxide semiconductor layer 112 may be formed under the same conditions for the first region 112-1, and the second region 112-2 may be formed by a water vapor process or an N₂O plasma oxidation process. The water vapor process may be performed at a temperature of 350° C. to 500° C. at a partial pressure of water vapor of 10% to 50% in a nitrogen atmosphere. The second region 112-2 may be formed by a fluorine doping process by use of NF₃ gas or SiF₄ gas, an Sn doping process, an Si doping process, a W doping process, instead of by using different partial pressures of oxygen at the time of forming the oxide semiconductor layer 112. The second region 112-2, in the case of being formed in such a manner, also has a carrier concentration lower than that of the first region 112-1. Alternatively, for example, the first region 112-1 may be formed of an In-Ga-Sn-O-based oxide material and the second region 112-2 may be formed of a Ga₂O₃-based oxide material, so that two regions containing different elements are formed. With such a method, in this embodiment, the second region 112-2 of the oxide semiconductor layer 112 has a carrier concentration lower than that of the first region 112-1.

Then, the oxide semiconductor layer 112 including the first region 112-1 and the second region 112-2 is patterned by etching. Therefore, as shown in FIG. 6B, an end of the first region 112-1 of the oxide semiconductor layer 112 is exposed. The oxide semiconductor layer 112 is not limited to this. For example, the second region 112-2 may be formed after the first region 112-1 of the oxide semiconductor layer 112 is formed and patterned. In this case, the end of the first region 112-1 may be covered with the second region 112-2. Such a structure further improves the physical characteristics of the oxide semiconductor layer 112.

FIG. 7A shows a stage of forming the first insulating layer 114 and a fourth conductive film 115 on the oxide semiconductor layer 112. The first insulating layer 114 is formed in substantially the same manner as the second insulating layer 106. The fourth conductive film 115 is formed in substantially the same manner as the first conductive film 103. Then, the fourth conductive film 115 is etched to form the first gate electrode 116. Thus, the transistor 110 a shown in FIG. 7B is manufactured.

According to the method of manufacturing the transistor 100 a in this embodiment, a multi-gradation photomask is used to decrease the number of photomasks required to manufacture the transistor 100 a. The use of the multi-gradation photomask allows a plurality patterns (the first oxide conductive layer 108 a, the second oxide conductive layer 108 b, the first line 110 a and the second line 110 b) to be formed by performing exposure merely once. This increases the productivity of integrated circuit elements each including the transistor 100 a and also decreases the production cost.

As shown in FIG. 7A and FIG. 7B, neither the first line 110 a nor the second line 110 b overlaps the first gate electrode 116. The first line 110 a and the second line 110 b are located as far as possible from the channel region of the transistor 100 a (region where the first gate electrode 116 overlaps the oxide semiconductor layer 112), so that the channel region is prevented from being contaminated with a metal element. For example, copper (Cu), which may be used as a material of the first line 110 a and the second line 110 b, is a killer impurity to the oxide semiconductor, which is an n-type semiconductor (impurity that deteriorates the characteristics of the oxide semiconductor and destroys the oxide semiconductor). In this embodiment, the first line 110 a and the second line 110 b are located as far as possible from the channel region of the transistor 110 a. Therefore, even if the first line 110 a and the second line 110 b contain copper (Cu), the oxide semiconductor layer 112 is suppressed from being contaminated with copper (Cu).

With the method of manufacturing the transistor 100 a in this embodiment, the first region 112-1 and the second region 112-2 of the oxide semiconductor layer 112 are formed continuously by using the same sputtering target and controlling the partial pressure of the sputtering gas. This improves the productivity.

Embodiment 2

In embodiment 2, an example of display device including transistors having substantially the same structure as that of the transistor described in embodiment 1 will be described. As shown in FIG. 8, a display device 120 includes a display region 121 including a plurality of pixels 122, a scanning line driving circuit 123, and a data line driving circuit 125. Although not shown in FIG. 8, the plurality of pixels 122 each include an organic EL element as a display element and a transistor driving the organic EL element.

2-1. Equivalent Circuit

FIG. 9 is an equivalent circuit diagram of each of the pixels 122 included in the display device 120 in this embodiment. The pixel 122 includes a selection transistor 124, a driving transistor 126, a capacitance element 128, and an organic EL element 130. The selection transistor 124 and the driving transistor 126 each have substantially the same structure as that of the transistor 100 a described in embodiment 1. Namely, FIG. 9 shows the transistors of a top-gate structure. The selection transistor 124 includes a first gate electrode 116 b, and the driving transistor 126 includes a first gate electrode 116 a.

In this embodiment, the selection transistor 124 and the driving transistor 126 are each of an n-channel type. The first gate electrode 116 b of the selection transistor 124 is connected with a gate signal line 132 a. One of input/output terminals (source and drain) of the selection transistor 124 is connected with a data signal line 134, and the other of the input/output terminals is connected with the first gate electrode 116 a of the driving transistor 126. The first gate electrode 116 a of the driving transistor 126 is connected with the other of the input/output terminals of the selection transistor 124. A drain of the driving transistor 126 is connected with the organic EL element 130, and a source of the driving transistor 126 is connected with a second common line 136 b. One of two terminals of the capacitance element 128 is connected with the other of the input/output terminals (source and drain) of the selection transistor 124. The other of the two terminals of the capacitance element 128 is connected with a first common line 136 a. The first common line 136 a and the second common line 136 b are supplied with, for example, a ground potential.

One of two terminals of the organic EL element 130 is connected with the drain of the driving transistor 126, and the other of the two terminals of the organic EL element 130 is connected with a power supply line 138. The power supply line 138 is supplied with a power supply potential VDD, which is higher than the potential of each of the common lines 136 a and 136 b. In this embodiment, the terminal of the organic EL element 130 that is connected with the drain of the driving transistor 126 is a cathode electrode, and the terminal of the organic EL element 130 that is connected with the power supply line 138 is an anode electrode.

2-2. Pixel Structure

FIG. 10 shows an example of planar structure of a pixel 122 a corresponding to the equivalent circuit shown in FIG. 9. FIG. 11A shows a cross-sectional structure taken along line A1-A2 in FIG. 10, and FIG. 11 B shows a cross-sectional structure taken along line B1-B2 in FIG. 10. FIG. 11A shows a cross-sectional structure of the driving transistor 126 and the organic EL element 130. FIG. 11B shows a cross-sectional structure of the selection transistor 124 and the capacitance element 128. The following description will be made with reference to FIG. 10, FIG. 11A and FIG. 11B optionally. In the plan view of the pixel 122 a shown in FIG. 10, the structure of the organic EL element 130 is omitted.

2-2-1. Driving Transistor

The driving transistor 126 has substantially the same structure as that of the transistor 100 a described in embodiment 1. Specifically, the driving transistor 126 includes the second insulating layer 106, a first oxide semiconductor layer 112 a (the first region 112-1 and the second region 112-2), the first insulating layer 114, and the first gate electrode 116 a, which are stacked. The first gate electrode 116 a is provided on a top surface of the first insulating layer 114 (surface opposite to the surface facing the substrate 102).

The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are provided between the second insulating layer 106 and the first oxide semiconductor layer 112 a. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are provided in contact with the first region 112-1 of the first oxide semiconductor layer 112 a.

The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are provided to partially overlap the first gate electrode 116 a and to hold the first gate electrode 116 a from both of two sides as seen in a plan view. The second oxide conductive layer 108 b includes patterns curved in a U shape (U-shaped curved patterns) as seen in a plan view in at least a region thereof overlapping the first gate electrode 116 a. The first oxide conductive layer 108 a includes linear patterns extending inward of the U-shaped curved patterns of the second oxide conductive layer 108 b. The cross-sectional view of the transistor 100 a shown in FIG. 1 shows a unit structure from the first oxide conductive layer 108 a to the second oxide conductive layer 108 b. A cross-sectional view of the driving transistor 126 shown in FIG. 10 taken along a second direction (D2 direction) includes a repetition of the cross-sectional view shown in FIG. 1.

The linear patterns of the first oxide conductive layer 108 a each have a width preferably in the range of 1.0 μm to 5 μm, and more preferably in the range of 1.5 μm to 3 μm. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b in the region overlapping the first gate electrode 116 a have a shortest distance therebetween that is preferably in the range of 1.5 μm to 10 μm, and is more preferably in the range of 2 μm to 5 μm. A structure in which the shortest distance between the first oxide conductive layer 108 a and the second oxide conductive layer 108 b is longer than, or equal to, the width of the linear pattern of the first oxide conductive layer 108 a may be adopted, so that the leak current caused when the driving transistor 126 is turned off is suppressed from being increased. With such a structure of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b, the gate-drain capacitance is suppressed from being fluctuated even if the first oxide conductive layer 108 a and the second oxide conductive layer 108 b is positionally shifted from the first gate electrode 116 a. Therefore, the display non-uniformity is suppressed to improve the yield.

In the driving transistor 126, the first oxide conductive layer 108 a, or a region of the first oxide conductive layer 108 a in contact with the first oxide semiconductor layer 112 a, is a drain region. The second oxide conductive layer 108 b, or a region of the second oxide conductive layer 108 b in contact with the first oxide semiconductor layer 112 a, is a source region.

The second oxide conductive layer 108 b of the driving transistor 126 is electrically connected with the first oxide semiconductor layer 112 a, the first common line 136 a and the second common line 136 b. The second common line 136 b is provided in the same layer structure as that of the data signal line 134 provided between the oxide conductive layer 108 c and the oxide semiconductor layer 112 b. The first common line 136 a and the second oxide conductive layer 108 b are electrically connected with each other via a first contact hole 117 a formed in the second insulating layer 106. The second common line 136 b is in direct contact with a top surface of the second oxide conductive layer 108 b.

The second insulating layer 106 includes, for example, a first silicon nitride film 141 a and a first silicon oxide film 140 a stacked in this order from the substrate 102 side. The first insulating layer 114 includes a second silicon oxide film 140 b and a second silicon nitride film 141 b stacked in this order from the first oxide semiconductor layer 112 a side.

A channel of the driving transistor 126 is formed in a region where the first oxide semiconductor layer 112 a overlaps the first gate electrode 116 a. Therefore, the first oxide semiconductor layer 112 a is provided in contact with the silicon oxide films 140 a and 140 b in the region where the channel is formed. The first oxide semiconductor layer 112 a is provided in contact with the insulating oxide films, and thus generation of oxygen deficiency in the first oxide semiconductor layer 112 a is suppressed. It is desired that the silicon oxide films 140 a and 140 b do not have oxygen deficiency so as not to draw out oxygen from the first oxide semiconductor layer 112 a. Rather, it is preferred that the silicon oxide films 140 a and 140 b contain an excessive amount of oxygen. A reason for this is that the silicon oxide films 140 a and 140 b, in the case of containing an excessive amount of oxygen, may be a source of oxygen for the first oxide semiconductor layer 112 a. The “silicon oxide film containing an excessive amount of oxygen” encompasses a silicon oxide film containing an excessive amount of oxygen with respect to the chemical stoichiometric composition, and also encompasses a silicon oxide film containing an excessive amount of oxygen in a lattice thereof. The second insulating layer 106 and the first insulating layer 114 may be formed of silicon oxide nitride or aluminum oxide instead of silicon oxide.

The driving transistor 126 is covered with a flattening layer 142. The flattening layer 142 is formed of an organic resin material such as, for example, an acrylic resin, a polyimide resin, an epoxy resin, a polyamide resin or the like. The flattening layer 142 is formed as follows. A composition containing a precursor of an organic resin material is applied, and a surface of the film formed by the composition is flattened by a leveling action of the film.

An opening 144 is provided in the flattening layer 142 and the first insulating layer 114. A first electrode 146, which is the cathode electrode of the organic EL element 130, is located to overlap the opening 144. The organic EL element 130 is formed of a plurality of layers stacked in the region of the opening 144.

In this embodiment, as described above with reference to FIG. 1, in the driving transistor 126, the oxide semiconductor layer 112 includes the first region 112-1 and the second region 112-2. The second region 112-2 has a carrier concentration lower than that of the first region 112-1. With such a structure, the driving transistor 126 includes a channel formed in the first region 112-1, of the oxide semiconductor layer 112, separated from the first insulating layer 114. In one embodiment according to the present invention, such a buried channel may be formed in order to manufacture a TFT operating in a normally-off mode (enhancement mode).

If the oxide semiconductor layer 112 does not include a region corresponding to the second region 112-2, a region of a poor quality (having a low density and having many defects) that is formed on the final stage of formation of the oxide semiconductor layer 112 is in direct contact with the first insulating layer 114. With such a structure, the channel region is formed to include the poor-quality region of the oxide semiconductor layer 112. Therefore, the field effect mobility is decreased. The poor-quality region is easily reduced during the formation of the first insulating layer 114. This significantly fluctuates the threshold voltage.

By contrast, in the driving transistor 126 in this embodiment, the oxide semiconductor layer 112 includes the second region 112-2 provided between the first region 112-1 and the first insulating layer 114. With such a structure, the field effect mobility is increased. In addition, the threshold voltage of the driving transistor 126 is suppressed from being fluctuated, and thus the driving transistor 126 has a higher reliability because of stable electric characteristics.

2-2-2. Selection Transistor

The selection transistor 124 has substantially the same structure as that of the transistor 100 a described in embodiment 1. Specifically, the selection transistor 124 includes the second insulating layer 106, a second oxide semiconductor layer 112 b (the first region 112-1 and the second region 112-2), the first insulating layer 114, and the first gate electrode 116 b, which are stacked. A channel of the second transistor 124 is formed in a region where the second oxide semiconductor layer 112 b overlaps the first gate electrode 116 b.

A third oxide conductive layer 108 c and a fourth oxide conductive layer 108 d are provided between the second insulating layer 106 and the second oxide semiconductor layer 112 b. The third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d are provided in contact with the first region 112-1 of the second oxide semiconductor layer 112 b, and thus act as a source region and a drain region.

The third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d are provided to partially overlap the first gate electrode 116 b and to hold the first gate electrode 116 b from both of two sides as seen in a plan view. The third oxide conductive layer 108 c includes a U-shaped curved pattern as seen in a plan view in at least a region thereof overlapping the first gate electrode 116 b. The fourth oxide conductive layer 108 d includes a linear pattern extending inward of the U-shaped curved pattern of the third oxide conductive layer 108 c. The cross-sectional view of the transistor 100 a shown in FIG. 1 shows a unit structure from the first oxide conductive layer 108 a to the second oxide conductive layer 108 b. A cross-sectional view of the selection transistor 124 shown in FIG. 10 taken along the second direction (D2 direction) includes a repetition of the cross-sectional view shown in FIG. 1.

The linear pattern of the fourth oxide conductive layer 108 d has a width preferably in the range of 1.0 μm to 5 μm, and more preferably in the range of 1.5 μm to 3 μm. The third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d in the region overlapping the first gate electrode 116 b have a shortest distance therebetween that is preferably in the range of 1.5 μm to 10 μm, and is more preferably in the range of 2 μm to 5 μm. A structure in which the shortest distance between the third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d is longer than, or equal to, the width of the linear pattern of the fourth oxide conductive layer 108 d may be adopted, so that the leak current caused when the selection transistor 124 is turned off is suppressed from being increased. With such a structure of the third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d, the gate-drain capacitance is suppressed from being fluctuated even if the third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d are positionally shifted from the first gate electrode 116 b. Therefore, the display non-uniformity is suppressed to improve the yield.

The third oxide conductive layer 108 c is electrically connected with the data signal line 134. The data signal line 134 is provided in the same layer structure as that of the line layer 110 provided between the oxide conductive layer 108 and the oxide semiconductor layer 112 described in the embodiment 1. The data signal line 134 is in direct contact with a top surface of the third oxide conductive layer 108 c. The second oxide semiconductor layer 112 b extends to a region where the data signal line 134 is located and covers the data signal line 134. The data signal line 134 is in direct contact with the third oxide conductive layer 108 c and thus has a larger contact area size than in the case where the data signal line 134 is connected with the third oxide conductive layer 108 c via a contact hole. Therefore, the contact resistance is decreased. A top surface and side surfaces of the data signal line 134 are covered with the second oxide semiconductor layer 112 b, and thus the data signal line 134 is not exposed to an oxidizing atmosphere or a reducing atmosphere during the production of the display device 120. Therefore, the data signal line 134 suppresses the resistance at a surface thereof from being increased.

2-2-3. Capacitance Element

The capacitance element 128 includes a first capacitance electrode 160 a, the second insulating layer 106, the fourth oxide conductive layer 108 d, and a second capacitance electrode 160 b. The second capacitance electrode 160 b is provided in the same layer structure as that of the data signal line 134. The fourth oxide conductive layer 108 d is electrically connected with the second capacitance electrode 160 b, and thus substantially acts as the electrode of the capacitance element 128.

The second oxide semiconductor layer 112 b and the first insulating layer 114 are provided above the second capacitance electrode 160 b. The second capacitance electrode 160 b is electrically connected with the first gate electrode 116 a via a second contact hole 117 b running through the first insulating layer 114 and the second oxide semiconductor layer 112 b.

2-2-4. Organic EL Element

The organic EL element 130 includes the first electrode 146 corresponding to the cathode electrode, the first oxide semiconductor layer 112 a (the first region 112-1 and the second region 112-2), an electron transfer layer 148, an electron injection layer 150, a light emitting layer 152, a hole transfer layer 154, a hole injection layer 156, and a second electrode 158 corresponding to the anode electrode, which are stacked from the substrate 102 side. A structure of an organic EL element in which a hole transfer layer, a light emitting layer, an electron transfer layer, and a cathode electrode are stacked in this order from the side of the anode electrode close to the substrate is referred to as a “normal stack structure”. In the organic EL element 130 in this embodiment, the electron transfer layer 148, the light emitting layer 152, the hole transfer layer 154 and the like are stacked in this order from the side of the cathode electrode close to the substrate 102. This structure is referred to as an “inverted stack structure”.

In this embodiment, the electron transfer layer 148, the second region 112-2 of the first oxide semiconductor layer 112 a, the first region 112-1 of the first oxide semiconductor layer 112 a and the first electrode 146 have energy levels decreasing in this order. Such a structure decreases the difference in the energy level. Thus, the electron injection efficiency is improved to improve the light emission efficiency of the organic EL element 130. In this embodiment, the driving transistor 126 is of an n-channel type. Therefore, if the organic EL element has a normal stack structure, the source is connected with the anode electrode. In this case, there is a problem that the level of drain current of the driving transistor is changed in accordance with the change in the characteristics of the organic EL element. However, in the case where the organic EL element 130 has an inverted stack structure as in this embodiment, the drain of the n-channel type driving transistor 126 is connected with the cathode of the organic EL element 130. Therefore, a circuit configuration in which the drain current is not much influenced by the change in the characteristics of the organic EL element 130 is provided.

On a top surface of the flattening layer 142 and in the opening 144 provided in the flattening layer 142 and the first insulating layer 114, the electron transfer layer 148, the electron injection layer 150, the light emitting layer 152, the hole transfer layer 154, the hole injection layer 156, and the second electrode 158 acting as the anode electrode are stacked. A region where a stacked body including these elements overlaps the first electrode 146 corresponding to the cathode electrode is a light emitting region of the organic EL element 130.

The organic EL element 130 in this embodiment is of a so-called bottom emission type, which outputs light toward the substrate 102. Hereinafter, each of the layers included in the organic EL element 130 will be described in detail.

2-2-4-1. Cathode Electrode

As a material of a cathode electrode of an organic EL element, an aluminum-lithium alloy (AlLi), a magnesium-silver alloy (MgAg) or the like is conventionally used. However, these materials are easily deteriorated by the influence of oxygen or moisture in the air, and thus are difficult to handle. These materials for the cathode electrode are metal materials, and thus are not suitable to an organic EL element that has an inverted stack structure and is of a bottom emission type.

In the organic EL element 130 in this embodiment, the first electrode 146, which is a cathode electrode, is formed of a transparent conductive material, and thus a bottom emission type structure is realized for the organic EL element 130. Specifically, the first oxide conductive layer 108 a of the driving transistor 126 extends to the region of the organic EL element 130 to act as the first electrode 146, which is a cathode electrode. With such an arrangement, the driving transistor 126 and the organic EL element 130 are electrically connected with each other with a simple structure. For example, in the case where an interlayer insulating layer is provided between the driving transistor and the organic EL element, a contact hole needs to be provided to connect the driving transistor and the organic EL element. By contrast, the structure of the first pixel 122 a in this embodiment does not require a contact hole.

The first electrode 146 as the cathode electrode is formed of the same conductive film as that of the first oxide conductive layer 108 a. The first oxide conductive layer 108 a is formed of a metal oxide material, a metal nitride material, or a metal oxide nitride material, all of which are conductive. A conductive film formed of such a material has a bandgap of 2.8 eV or larger, preferably of 3.0 eV or larger, and therefore transmits almost all of light of a visible region. Therefore, such a material is usable for an electrode on the light output side of the organic EL element 130.

On a top surface of the first electrode 146 corresponding to the cathode electrode, the first oxide semiconductor layer 112 a extending from the driving transistor 126 may be provided. The first oxide semiconductor layer 112 a has a bandgap of 3 eV or larger, and thus is visible light-transmissive. As described below, in this embodiment, the electron transfer layer 148 is formed of a metal oxide. The first oxide semiconductor layer 112 a formed of the same material, or the same type of material, as the electron transfer layer 148 is located between the electron transfer layer 148 and the first electrode 146 corresponding to the cathode electrode, so that formation of an electron injection barrier is prevented. In other words, the first oxide semiconductor layer 112 a extending from the channel region of the driving transistor 126 may be used as a part of the electron transfer layer 148 in contact with the first electrode 146 corresponding to the cathode electrode.

2-2-4-2. Electron Transfer Layer

The electron transfer layer 148 is formed of a metal oxide material. Examples of the metal oxide material usable for the electron transfer layer 148 include substantially the same materials described in embodiment 1, specifically, a four-component oxide material, a three-component oxide material, a two-component oxide material, and a one-component oxide material. These metal oxide materials may be in an amorphous state, a crystalline state or a mixed phase of an amorphous state and a crystalline state. The electron transfer layer 148 is formed of, for example, one or a plurality of materials selected from an oxide of indium, an oxide of zinc, an oxide of gallium (Ga), and an oxide of tin (Sn). Such a metal oxide material should not absorb visible light and needs to be transparent, and thus is required to have a bandgap of 3.0 eV or larger. The electron transfer layer 148 may have a maximum possible thickness to prevent short circuiting between the anode electrode and the cathode electrode. This significantly improves the yield of the organic EL panel. A representative material of the electron transfer layer 148 is ZnSiO_(x). ZnO is doped with SiO₂ at 10 atm % to 15 atm % to provide physical properties suitable for the electron transfer layer 148, namely, a bandgap of 3.5 eV and a work function of 3.5 eV. An MgZnO_(x)-based oxide semiconductor containing Mg at 20 atm % and Zn at 80 atm % may also provide substantially the same physical properties. The electron transfer layer 148 having such a composition may be formed by sputtering, vacuum vapor deposition, coating or the like. The electron transfer layer 148 is formed by such a method to have a thickness of 50 nm to 1000 nm.

It is preferred that the electron transfer layer 148 has a carrier concentration that is 1/10 or less, preferably 1/100 or less, of the average carrier concentration of the first oxide semiconductor layer 112 a. In other words, the average carrier concentration of the first oxide semiconductor layer 112 a is at least 10 times, preferably 100 times, the carrier concentration of the electron transfer layer 148. Specifically, the carrier concentration of the electron transfer layer 148 is 10¹³/cm³ to 10¹⁷/cm³, whereas the carrier concentration of the first region 112-1 of the first oxide semiconductor layer 112 a is 1×10¹⁵/cm³ to 1×10¹⁹/cm³. The difference between the carrier concentrations of the electron transfer layer 148 and the first region 112-1 of the first oxide semiconductor layer 112 a may be at least one digit, preferably at least two digits. The first region 112-1 of the first oxide semiconductor layer 112 a has a carrier concentration of is 10¹⁵/cm³ to 10¹⁹/cm³, and thus decreases the resistance loss in the electric connection between the driving transistor 126 and the organic EL element 130, and suppresses the driving voltage from being increased. In the case where the carrier concentration of the electron transfer layer 148 is 10²⁰/cm³ or higher, the excited state in the light emitting layer 152 is deactivated and the light emission efficiency is decreased. By contrast, in the case where the carrier concentration of the electron transfer layer 148 is 10¹³/cm³ or lower the number of carriers supplied to the light emitting layer 152 is decreased and thus a sufficient level of luminance is not provided. As described above, the first oxide conductive layer 108 a and the first oxide semiconductor layer 112 a extending from the driving transistor 126 are provided to stack on, and to be in contact with, the electron transfer layer 148, and the carrier concentrations of the three layers are made different from each other. With such a structure, the driving voltage is prevented from being increased, and the electron injection efficiency is improved. As a result, the light emission efficiency of the organic EL element 130 is improved.

2-2-4-3. Electron Injection Layer

In an organic EL element, an electron injection layer is used in order to decrease the energy barrier and thus to inject the electrons from the cathode electrode into an electron transfer material. In this embodiment, the electron injection layer 150 is used in order to allow the electrons to be injected easily from the electron transfer layer 148 formed of an oxide semiconductor into the light emitting layer 152. Thus, the electron injection layer 150 is provided between the electron transfer layer 148 and the light emitting layer 152.

It is desired that the electron injection layer 150 is formed of a material having a small work function in order to allow the electrons to be injected easily into the light emitting layer 150 formed of an organic material. The electron injection layer 150 is formed of an oxide of calcium (Ca) and an oxide of aluminum (Al). The electron injection layer 150 is preferably formed of, for example, C12A7 (12CaO.7Al₂O₃) electride. C12A7 (12CaO.7Al₂O₃) electride has semiconductor characteristics, is controllable to have a desired level of resistance from a high resistance to a low resistance, and has a work function of 2.4 eV to 3.2 eV, which is about the same as that of an alkaline metal. For these reasons, C12A7 (12CaO.7Al₂O₃) electride is preferably usable for the electron injection layer 150.

The electron injection layer 150 of C12A7 electride is formed by sputtering by use of a polycrystal of C12A7 electride as a target. C12A7 electride has semiconductor characteristics, and thus the electron injection layer 150 may be formed to have a thickness of 1 nm to 100 nm. Regarding C12A7 electride, it is preferred that the molar ratio of Ca:Al is in the range of 13:13 to 11:16. C12A7 electride is formed by sputtering and thus is preferably amorphous. Alternatively, C12A7 electride may be crystalline.

C12A7 electride is stable in the atmosphere, and thus has an advantage of being easier to handle than an alkaline metal compound conventionally used for an electron injection layer such as lithium fluoride (LiF), lithium oxide (Li₂O), sodium chloride (NaCl), potassium chloride (KCl), or the like. Use of C12A7 electride makes it unnecessary to work in dry air or inactive gas during the formation of the organic EL element 130. The conditions for the formation of the organic EL element 130 are alleviated.

C12A7 electride has a large ionization potential. Therefore, the electron injection layer 150, when being located to face the hole transfer layer 154 while having the light emitting layer 152 between the electron injection layer 150 and the hole injection layer 154 therebetween, acts as a hole block layer. Namely, the electron injection layer 150 formed of C12A7 electride is provided between the electron transfer layer 148 and the light emitting layer 152, so that holes injected into the light emitting layer 152 are suppressed from running to the first 146 as the cathode electrode, and thus the light emission efficiency is improved. MgZnO_(x) (containing Mg at 30 atm % and Zn at 70 atm %) is also usable for the electron injection layer 150.

2-2-4-4. Light Emitting Layer

The light emitting layer 152 may be formed of any of various materials. For example, the light emitting layer 152 may be formed of a fluorescent compound emitting fluorescence or phosphorescent compound emitting phosphorescence.

Examples of light emitting material emitting blue light usable for the light emitting layer 152 include N,N′-bis[4-(9H-carbazole-9-yl)phenyl-N,N′-diphenylstilbene-4,4′-diamine (YGAS2S), 4-(9H-carbazole-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine (YGAPA), and the like. Examples of light emitting material emitting green light usable for the light emitting layer 152 include N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazole-3-amine (2PCAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazole-3-amine (2PCABPhA), N-(9,10-diphenyl-2-anthryl)-N,N′,N-triphenyl-1,4-phenylenediamine (2DPAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine (2DPABPhA), N-[9.10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazole-9-yl)phenyl]-N-phenylanthracene-2-amine (2YGABPhA), N,N,9-triphenylanthracene-9-amine (DPhAPhA), and the like. Examples of light emitting material emitting red light usable for the light emitting layer 152 include N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (p-mPhTD), 7,13-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine (p-mPhAFD), and the like. A phosphorescent material such as bis[2-(2′-benzo[4,5-α]thienyl)pyridinatho-N,C^(3′)]iridium(III)acetylacetonate (Ir(btp)₂(acac)) or the like is also usable.

The light emitting layer 152 may be formed of any of various known materials other than the above-listed materials. The light emitting layer 152 may be formed by vapor deposition, transfer, spin-coating, spray-coating, gravure printing, or the like. The light emitting layer 152 may have an optionally selected thickness, and has a thickness of, for example, 10 nm to 100 nm.

2-2-4-5. Hole Transfer Layer

The hole transfer layer 154 is formed of a material having hole transferability. The hole transfer layer 154 is formed of, for example, an arylamine-based compound, an amine compound containing a carbozole group, an amine compound containing a fluorene derivative, or the like. Examples of materials usable for the hole transfer layer 154 include organic materials such as 4,4′-bis[N-(naphtyl)-N-phenyl-amino]biphenyl (α-NPD), N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD), 2-TNANA, 4,4′,4″-tris(N-(3-methylphenyl)N-phenylamino)triphenylamine (MTDATA), 4,4′-N,N′-dicarbazolebiphenyl (CBP), 4,4′-bis[N-(9,9-dimethylfluorene-2-yl)-N-phenylamino]biphenyl (DFLDPBi), 4,4′bis[N-(spiro-9,9′-bifluorene-2-yl)-N-phenylamino]biphenyl (BSPB), spiro-NPD, spiro-TPD, apiro-TAD, TNB, and the like.

The hole transfer layer 154 is formed of a common film formation method such as vacuum vapor deposition, coating or the like. The hole transfer layer 154 is formed by such a method to have a thickness of 10 nm to 500 nm. The hole transfer layer 154 may be omitted.

2-2-4-6. Hole Injection Layer

The hole injection layer 156 is formed of a material having a high capability of injecting holes into an organic layer. Examples of materials having a high capability of injecting holes and usable for the hole injection layer 156 include metal oxides such as an oxide of molybdenum, an oxide of vanadium, an oxide of ruthenium, an oxide of tungsten, an oxide of manganese and the like. Examples of materials having a high capability of injecting holes and usable for the hole injection layer 156 also include organic compounds such as phthalocyanine (H₂Pc), copper (II) phthalocyanine (CuPc), vanadylphthalocyanine (VoPc), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (TDATA), 4,4′,4″-tris(N-(3-methylphenyl)-N-phenylamino]triphenylamine (MTDATA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (DPAB), 4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino) biphenyl (DNPTD), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (DPA3B), 3-[N-(9-phenylcarbozole-3-yl)-N-phenylamino]-9-phenylcarbazole (PCzPCA1), 3,6-bis[N-(9-phenylcarbazole-3-yl)-N-phenylamino]-9-phenylcarbazole (PCzPCA2), 3-[N-(1-naphtyl)-N-(9-phenylcarbazole-3-yl)amino]-9-phenylcarbazole (PCzPCN1), 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (HAT-CN), and the like.

The hole injection layer 156 is formed of a common film formation method such as vacuum vapor deposition, coating or the like. The hole injection layer 156 is formed by such a method to have a thickness of 1 nm to 100 nm.

2-2-4-7.Anode Electrode

The second electrode 158 corresponding to the anode electrode is formed of a metal material, an alloy or a conductive compound having a high work function (specifically, 4.0 eV or larger). The second electrode 158 corresponding to the anode electrode is formed of, for example, indium tin oxide (ITO), indium zinc oxide (IZO), indium oxide containing tungsten oxide and zinc oxide (IWZO), or the like. The second electrode 158 corresponding to the anode electrode of such a conductive metal oxide material is formed by vacuum vapor deposition or sputtering. In this embodiment, the organic EL element 130 is of a bottom emission type. Therefore, it is preferred that the second electrode 158 corresponding to the anode electrode is light reflective or has a light reflective surface. A film of a conductive metal oxide such as indium tin oxide (ITO), indium zinc oxide (IZO) or the like is light-transmissive. Therefore, the second electrode 158 may include a metal film of aluminum (Al), silver (Ag) or the like at a surface thereof that is opposite to a surface facing the hole injection layer 156. Although not shown in FIG. 10, FIG. 11A and FIG. 11B, a passivation layer blocking transmission of oxygen (O₂) or moisture (H₂O) may be provided on the second electrode 158 corresponding to the anode electrode in substantially the entirety of the display region 121.

As described above, in this embodiment, the pixel 122 a in which the driving transistor 126 exhibiting n-channel type conductivity and the organic EL element 130 are electrically connected with each other is realized. In this case, the organic EL element 130 may have an inverted stack structure, in which the electron transfer layer 148, the electron injection layer 150, the light emitting layer 152, the hole transfer layer 154, the hole injection layer 156 and the like are stacked appropriately from the side of the first electrode 146, which is the cathode electrode. Since the first electrode 146, which is the cathode electrode, does not need to be formed of an alkaline metal material, the reliability of the display device 120 is improved. In addition, the electron transfer layer 148 and the electron injection layer 150, which are located in lower layers, are formed of an inorganic insulating material. Therefore, even if an organic layer is formed on these inorganic insulating layers, the characteristics are suppressed from decreasing by denaturing or the like. Thus, the characteristics of the organic EL element 130 are stabilized.

2-3. Transistor Structure

As shown in FIG. 11A and FIG. 11B, the pixel 122 a in this embodiment has a structure in which the second electrode 158 covers the entire surfaces of the driving transistor 126 and the selection transistor 124. The driving transistor 126 and the selection transistor 124 each have a bottom-contact top-gate structure; specifically, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located in contact with a bottom surface of the oxide semiconductor layer 112, and the first gate electrode 116 is located above the oxide semiconductor layer 112.

FIG. 12A shows a bottom-contact bottom-gate type transistor 300 having the following cross-sectional structure. The transistor 300 includes a gate electrode 304, a second insulating layer 306, a first oxide conductive layer 308 a, a second oxide conductive layer 308 b, a first line 310 a, a second line 310 b, an oxide semiconductor layer 312, a first insulating layer 314, a flattening layer 342, and an anode electrode 358, which are stacked in this order on a substrate 302. FIG. 12B shows a top-contact bottom-gate type transistor 400 having the following cross-sectional structure. The transistor 400 includes a gate electrode 404, a second insulating layer 406, an oxide semiconductor layer 412, a first oxide conductive layer 408 a, a second oxide conductive layer 408 b, a first line 410 a, a second line 410 b, a first insulating layer 414, a flattening layer 442, and an anode electrode 458, which are stacked in this order on a substrate 402. In each of the bottom-gate type transistor 300 and the bottom-gate type transistor 400 having the above-described structures, an electric current flows in an area, of the first region 312-1 or 412-1 of the oxide semiconductor layer 312 or 412, that is closer to the first gate electrode 304 or 404 (area represented by the dashed line). Also in each of the bottom-gate type transistor 300 and the bottom-gate type transistor 400, the back channel side (the side of the oxide semiconductor layer 312 or 412 facing the anode electrode 358 or 458) is easily influenced by the anode electrode 358 or 458. Specifically, the anode electrode 358 or 458 has a positive potential, and an interface between the oxide semiconductor layer 312 or 412 and the second insulating layer 314 or 414 (back channel interface), and the anode electrode 358 or 458, are separated from each other by an interval of about 3 μm to about 5 μm. Therefore, positive charges are easily accumulated at the interface between the back channel side of the oxide semiconductor layer 312 or 412 and the first insulating layer 314 or 414. When the positive charges are accumulated at the interface between the back channel side and the first insulating layer 314 or 414, there occurs a problem that the threshold voltage of the transistor 300 or 400 is shifted to a negative side (the transistor 300 or 400 becomes a normally-off transistor).

In order to solve such a problem, it is preferred that gate electrode is provided above the oxide semiconductor layer 112 as in this embodiment. In this case, the first gate electrode 116 may be grounded to have a constant potential, so that the potential on the back channel side is stabilized.

FIG. 13 shows a bottom-contact top-gate structure of the transistor 100 a in this embodiment. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located in contact with the bottom surface of the oxide semiconductor layer 112, and the first gate electrode 116 is located above the oxide semiconductor layer 112. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are in contact with the first region 112-1 of the oxide semiconductor layer 112 having a carrier concentration higher than that of the second region 112-2, so that the contact resistance is decreased. With such a structure, the transistor 100 a in this embodiment has a higher admittance and a higher level of drain current.

FIG. 13 shows an embodiment of the transistor 100 a. In this embodiment, the transistor 100 a has a structure in which the first gate electrode 116 above the oxide semiconductor layer 112 overlaps both of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b, which correspond to the source and drain electrodes. The first gate electrode 116 having width W_(top) in a channel length direction overlaps each of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b by width W_(ov). Since the first gate electrode 116 partially overlaps both of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b, the channel region in the oxide semiconductor layer 112 is substantially blocked against the electric field of the second electrode 158. Therefore, even if the second electrode 158 is located to cover the entire surface of the transistor 100 a, the channel region is not influenced by the electric field of the second electrode 158. Thus, the threshold voltage of the transistor 100 a is prevented from being fluctuated along with time.

2-4. Method of Manufacturing the Display Device

An example of method of manufacturing the display device 120 in an embodiment according to the present invention will be described. In the following description, the same explanations as those on the method of manufacturing the transistor 100 a provided in embodiment 1 will be omitted, and only the differences will be provided.

FIG. 14, FIG. 15A and FIG. 15B show a stage of forming the first capacitance electrode 160 a and the first common line 136 a on the substrate 102, and a stage of forming the second insulating layer 106. FIG. 14 is a plan view of a region corresponding to one pixel 122 a. FIG. 15A is a cross-sectional view taken along line A1-A2 in FIG. 14, and FIG. 15B is a cross-sectional view taken along line B1-B2 in FIG. 14.

As shown in FIG. 14, FIG. 15A and FIG. 15B, the first common line 136 a and the first capacitance electrode 160 a are formed of the same conductive film as each other. Therefore, the first common line 136 a and the first capacitance electrode 160 a are formed as one continuous pattern formed of a conductive film in the same layer.

The second insulating layer 106 is formed on a top surface of the first common line 136 a and the first capacitance electrode 160 a. For example, the second insulating layer 106 is formed by stacking the first silicon nitride film 141 a and the first silicon oxide film 140 a from the substrate 102 side. The first silicon nitride film 141 a is formed by ICP-CVD by use of gas such as SiF₄, N₂ or the like as source gas. The first silicon oxide film 140 a is also formed by plasma CVD by use of tetraisocyanatesilane-based material, N₂O or the like optionally. The second insulating layer 106 is formed on substantially the entire surface of the substrate 102.

FIG. 16A and FIG. 16B show a stage where the second conductive film 107 and the third conductive film 109 are formed on a top surface of the second insulating layer 106, and a resist mask 207 is formed thereon by use of the multi-gradation photomask 201. The photoresist film 205 is exposed to light through the transmissive region, the semi-transmissive region 202 and the non-transmissive region 203 of the multi-gradation photomask 201. As a result, three types of portions, specifically, an exposed portion, a gradation-exposed portion, and a non-exposed portion are formed in the photoresist film 205. In correspondence with the pattern of the non-transmissive region 203 of the multi-gradation photomask 201, resist masks 207 are formed such that regions thereof corresponding to regions, of the third conductive film 109, that are to become the second common line 136 b (FIG. 16A) and the data signal line 134 (FIG. 16B) are unexposed and thicker than the remaining region. The second conductive film 107 is formed of a transparent conductive material, and the third conductive film 109 is formed of a metal material. As shown in FIG. 16A, the first contact hole 117 a is formed in advance in the second insulating layer 106 in order to expose the first common line 136 a.

FIG. 17, FIG. 18A and FIG. 18B show a state where the third conductive film 109 and the second conductive film 107 are etched by use of the resist masks 207. FIG. 17 is a plan view of a region corresponding to one pixel 122 a. FIG. 18A is a cross-sectional view taken along line A1-A2 in FIG. 17, and FIG. 18B is a cross-sectional view taken along line B1-B2 in FIG. 17.

The first oxide conductive layer 108 a, the second oxide conductive layer 108 b, the third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d are formed of the second conductive film 107. The first oxide conductive layer 108 a, the second oxide conductive layer 108 b, the third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d are formed on the second insulating layer 106. The second common line 136 b, the second capacitance electrode 160 b and the data signal line 134 are formed of the third conductive film 109. The second common line 136 b is formed on the second oxide conductive layer 108 b. The second common line 136 b is formed in contact with the top surface of the second oxide conductive layer 108 b. In this state, the first common line 136 a, the second oxide conductive layer 108 b and the second common layer 136 b are electrically connected with each other.

The second capacitance electrode 160 b is formed in contact with a top surface of the fourth oxide conductive layer 108 d. The second capacitance electrode 160 b is located to at least partially overlap the first capacitance electrode 160 a while having the fourth oxide conductive layer 108 d and the second insulating layer 106 between the second capacitance electrode 160 b and the first capacitance electrode 160 a. The capacitance element 128 is formed in a region where the first capacitance electrode 160 a and the second capacitance electrode 160 b overlap each other while having the second insulating layer 106 therebetween.

The data signal line 134 is formed in contact with the top surface of the third oxide conductive layer 108 c. In this state, the third oxide conductive layer 108 c and the data signal line 134 are electrically connected with each other. The third oxide conductive layer 108 c is provided along the data signal line 134, and thus is electrically connected with the data signal line 134 with certainty.

An end of the second common line 136 b is located inner to an end of the second oxide conductive layer 108 b. With such an arrangement, even though the second oxide conductive layer 108 b and the second common line 136 b are stacked on each other, the ends thereof form a stepped portion. Therefore, the step coverage of the oxide semiconductor layer 112 and the first insulating layer 114 formed in a later stage is in a good state. Similarly, an end of the data signal line 134 is located inner to an end of the third oxide conductive layer 108 c, and an end of the second capacitance electrode 160 b is located inner to an end of the fourth oxide conductive layer 108 d. Therefore, the step coverage of the oxide semiconductor layer 112 and the first insulating layer 114, which are to be formed on these ends in a later stage, are in a good state.

FIG. 19, FIG. 20A and FIG. 20B show a stage of forming the oxide semiconductor layer 112, the first insulating layer 114 and the fourth conductive film 115. FIG. 19 is a plan view of a region corresponding to one pixel 122 a. FIG. 20A is a cross-sectional view taken along line A1-A2 in FIG. 19, and FIG. 20B is a cross-sectional view taken along line B1-B2 in FIG. 19.

The first oxide semiconductor layer 112 a is formed to cover substantially the entire surfaces of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b. The second oxide semiconductor layer 112 b is formed to cover substantially the entire surfaces of the third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d. The first oxide semiconductor layer 112 a and the second oxide semiconductor layer 112 b each have a stack structure including the first region 112-1 and the second region 112-2. The first oxide semiconductor layer 112 a and the second oxide semiconductor layer 112 b are formed as follows. The first region 112-1 and the second region 112-2 are formed in this order by sputtering by use of an oxide semiconductor as a target, and are subjected to a lithography step. As a result, the first oxide semiconductor layer 112 a and the second oxide semiconductor layer 112 b having a predetermined shape described above are formed. The first region 112-1 of the first oxide semiconductor layer 112 a is formed in contact with, and thus is electrically connected with, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b. The first region 112-1 of the second oxide semiconductor layer 112 b is formed in contact with, and thus is electrically connected with, the third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d.

The first insulating layer 114 is formed on a top surface of the first oxide semiconductor layer 112 a and the second oxide semiconductor layer 112 b. The first insulating layer 114 is formed by, for example, stacking the second silicon oxide film 140 b and the second silicon nitride film 141 b in this order from the oxide semiconductor layer 112 side. As a result, the first silicon oxide film 140 a is formed below the oxide semiconductor layer 112, and the second silicon oxide film 140 b is formed above the oxide semiconductor layer 112. The oxide semiconductor layer 112 is held between the oxide insulating films, and thus is suppressed from having a defect (donor level) caused thereto by oxygen deficiency.

It is desired that the silicon oxide films 140 a and 140 b do not have oxygen deficiency so as not to draw out oxygen from the first oxide semiconductor layer 112 a. Rather, it is even preferred that the first silicon oxide films 140 a and 140 b contain an excessive amount of oxygen. The first insulating layer 114, after being formed, is heat-treated at a temperature of 250° C. to 400° C., and thus oxygen is diffused from the first silicon oxide film 140 a and the second silicon oxide film 140 b to the first oxide semiconductor 112 a and the second oxide semiconductor layer 112 b. Since such heat treatment is performed, even if the oxide semiconductor layer 112 includes oxygen deficiency, the oxygen deficiency is compensated for by oxygen diffused from the silicon oxide films 140, and the defect, which would become a donor level, is extinguished. Therefore, the resistance is increased.

In the second layer 114, the second contact hole 117 b is formed in a region overlapping the second capacitance electrode 160 b. Then, the fourth conductive film 115 is formed. The fourth conductive film 114 is formed in substantially the same manner as the first conductive film 103.

FIG. 21, FIG. 22A and FIG. 22B show a stage of forming the first gate electrode 116. FIG. 21 is a plan view of a region corresponding to one pixel 122 a. FIG. 22A is a cross-sectional view taken along line A1-A2 in FIG. 21, and FIG. 22B is a cross-sectional view taken along line B1-B2 in FIG. 21.

The first gate electrode 116 is formed by a lithography step and an etching step performed on the fourth conductive film 115. The first gate electrode 116 a is formed to include a region overlapping an end part of each of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b, with the first insulating layer 114 being provided between the first gate electrode 116 a and the first oxide conductive layer 108 a/the second oxide conductive layer 108 b. The first gate electrode 116 b is formed to include a region overlapping an end part of each of the third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d, with the first insulating layer 114 being provided between the first gate electrode 116 a and the third oxide conductive layer 108 c/the fourth oxide conductive layer 108 d. As a result, the driving transistor 126 and the selection transistor 124 are formed. The capacitance element 128 is electrically connected with the first gate electrode 116 a via the contact hole 117 b.

As shown in FIG. 23A and FIG. 23B, the flattening layer 142 is formed to bury the selection transistor 124, the driving transistor 126 and the capacitance element 128. The flattening layer 142 is formed of an organic resin material such as, for example, an acrylic resin, a polyimide resin, an epoxy resin, a polyamide resin or the like. In the flattening layer 142, the opening 144 is formed in a region overlapping the first electrode 146, which is the cathode electrode, in order to expose the first oxide semiconductor layer 112 a. In the case where the flattening layer 142 is formed of a photosensitive resin material, the opening 144 is formed by exposure to light by use of a photomask. Before the flattening layer 142 is formed, an opening is formed in advance in the first insulating layer 114 in a region corresponding to the opening 144. Alternatively, an opening that exposes the first oxide semiconductor layer 112 a may be formed in the first insulating layer 114 in the step of forming the opening 144 in the flattening layer 142. It is preferred that the opening 144 in the flattening layer 142 has a tapered inner wall in order to allow the organic EL element 130 to be formed easily.

FIG. 24A and FIG. 24B show a stage of forming the electron transfer layer 148 and the electron injection layer 150. The electron transfer layer 148 is formed of a metal oxide material. Examples of the metal oxide material usable for the electron transfer layer 148 include substantially the same materials described in embodiment 1, specifically, a four-component oxide material, a three-component oxide material, a two-component oxide material, and a one-component oxide material. The electron transfer layer 148 is formed by sputtering by use of any of the above-listed materials as a sputtering target. The electron injection layer 150 is formed of C12A7 electride. The electron injection layer 150 may be formed by sputtering by use of C12A7 electride as a sputtering target. In this case, the sputtering may be performed by use of at least one type of gas selected from the group consisting of He (helium), Ne (neon), N₂ (nitrogen), Ar (argon), NO (nitrogen monoxide), Kr (krypton), and Xe (xenon). The electron transfer layer 148 and the electron injection layer 150 are used commonly in a plurality of the pixels 122 a, and therefore are formed on substantially the entirety of a region where the pixels 122 a are located.

Then, the light emitting layer 152, the hole transfer layer 154, the hole injection layer 156 and the second electrode 158 as the anode electrode are formed. As a result, the pixel shown in FIG. 11A and FIG. 11B is formed. The light emitting layer 152 is formed of different light emitting materials for red pixels, green pixels and blue pixels. In the case where light emitted from the light emitting layer 152 has a white light emission spectrum, the light emitting layer 152 may be formed in substantially the entirety of the display region 121 as a layer common to all the pixels 122 a. The hole transfer layer 154 and the hole injection layer 156 are each formed on substantially the entirety of the region where the pixels 122 a are located, as layers common to all the pixels 122 a. The second electrode 158 as the anode electrode is used as a common electrode to the pixels 122 a, and therefore is formed on substantially the entirety of the region where the pixels 122 a.

According to the method of manufacturing the display device 120 in this embodiment, the multi-gradation photomask 201 is used to decrease the number of photomasks required to manufacture the display device 120. FIG. 25A to FIG. 25C each show a pattern of a photomask usable to manufacture the display device 120, more specifically, a part of the photomask in the vicinity of the capacitance element 128 and the selection transistor 124. FIG. 25A shows the multi-gradation photomask 201 usable to form the third oxide conductive layer 108 c, the fourth oxide conductive layer 108 d, the data signal line 134 and the second capacitance electrode 160 b. The non-transmissive regions 203 of the multi-gradation photomask 201 correspond to the regions where the data signal 134 and the second capacitance electrode 160 b are to be formed. The semi-transmissive region 202 corresponds to the regions where the third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d are to be formed. By use of the multi-gradation photomask 201 having such a pattern, a plurality of patterns (the first oxide conductive layer 108 a, the second oxide conductive layer 108 b, the third oxide conductive layer 108 c, the fourth oxide conductive layer 108 d, the data signal line 134, the second common line 136 b, the second capacitance electrode 160 b, and the like) are formed by one cycle of exposure. FIG. 25B shows a photomask usable to form the second oxide semiconductor layer 112 b. By use of the photomask having such a pattern, a plurality of patterns (the first oxide semiconductor layer 112 a and the second oxide semiconductor layer 112 b) are formed by one cycle of exposure. FIG. 25C shows a photomask usable to form the first gate electrode 116. By use of the photomask having such a pattern, a plurality of patterns (the first gate electrode 116 a and the first gate electrode 116 b) are formed by one cycle of exposure. As a result, the productivity of the display device 120 is improved, and the production cost is decreased. In this embodiment, the transistors are described as thin film transistors of an organic EL display device. The selection transistor 124 is also applicable to a liquid crystal display device. The photomasks shown in FIG. 25A to FIG. 25C may be modified to form a selection transistor for a liquid crystal display device.

In this embodiment, the selection transistor 124 and the driving transistor 126 are both of a top-gate type. The present invention is not limited to this. For example, the selection transistor 124 and the driving transistor 126 may each be a dual-gate transistor, in which a bottom gate is located in a region overlapping the first gate electrode 116 as described below. The pixel circuit is not limited to the circuit shown in FIG. 9. The transistors and the organic EL element in this embodiment are applicable to a pixel circuit including three or more transistors in one pixel.

Embodiment 3

In embodiment 3, a transistor having a structure different from that in embodiment 1 will be described. In embodiment 3, unlike in embodiment 1, a second gate electrode 104 is located to face one surface of the oxide semiconductor layer 112 (surface facing the substrate 102). Components that are the same as those in embodiments 1 and 2 will bear the same reference signs, and the descriptions thereof will not be repeated.

3-1. Transistor Structure

FIG. 26 is a cross-sectional view showing a structure of a transistor 100 b in an embodiment according to the present invention. The transistor 100 b includes the second gate electrode 104, the second insulating layer 106, the oxide semiconductor layer 112, the first insulating layer 114, and the first gate electrode 116, which are provided on the substrate 102 having an insulating surface. The oxide semiconductor layer 112 includes the first region 112-1 and the second region 112-2 stacked from the substrate 102 side.

The second gate electrode 104 is located to face one surface of the oxide semiconductor layer 112 (surface facing the substrate 102). The second insulating layer 106 is located between the oxide semiconductor layer 112 and the second gate electrode 104. The first gate electrode 116 is located to face the other surface of the oxide semiconductor layer 112 (surface opposite to the surface facing the substrate 102). The first insulating layer 114 is located between the oxide semiconductor layer 112 and the first gate electrode 116. The second gate electrode 104 and the first gate electrode 116 are located to partially overlap each other while having the second insulating layer 106, the oxide semiconductor layer 112 and the first insulating layer 114 between the second gate electrode 104 and the first gate electrode 116. In the transistor 100 b, a channel is formed in a region where the oxide semiconductor layer 112 overlaps the second gate electrode 104 and the first gate electrode 116. The second insulating layer 106 acts as a gate insulating film in a region where the oxide semiconductor layer 112 and the second gate electrode 104 overlap each other. The first insulating layer 114 acts as a gate insulating film in a region where the oxide semiconductor layer 112 and the first gate electrode 116 overlap each other.

The second insulating layer 106 is located between the oxide semiconductor layer 112 and the second gate electrode 104. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located between the oxide semiconductor layer 112 and the second insulating layer 106. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are provided in contact with the oxide semiconductor layer 112. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located such that one end part of the first oxide conductive layer 108 a and one end part of the second oxide conductive layer 108 b overlap each of the second gate electrode 104 and the first gate electrode 116. One of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b acts as a source region, and the other of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b acts as a drain region. With the structure shown in FIG. 26, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located such that one end part of the first oxide conductive layer 108 a and one end part of the second oxide conductive layer 108 b overlap each of the second gate electrode 104 and the first gate electrode 116. Therefore, the oxide semiconductor layer 112 does not include any offset region (region having a high resistance). Thus, the level of on-current is increased.

The first line 110 a is provided in contact with the first oxide conductive layer 108 a, and the second line 110 b is provided in contact with the second oxide conductive layer 108 b. The first line 110 a is located between the first oxide conductive layer 108 a and the oxide semiconductor layer 112, and the second line 110 b is located between the second oxide conductive layer 108 b and the oxide semiconductor layer 112. The first line 110 a and the second line 110 b are respectively located in contact with the first oxide conductive layer 108 a and the second oxide conductive layer 108 b. This decreases the number of photolithography steps

3-2. Operations and Functions of the Transistor

In the transistor 100 b, the second gate electrode 104 is located to face one surface of the oxide semiconductor layer 112 (surface facing the substrate 102), and the first gate electrode 116 is located to face the other surface of the oxide semiconductor layer 112 (surface opposite to the surface facing the substrate 102). A channel is formed in the region where the oxide semiconductor layer 112 overlaps the second gate electrode 104 and the first gate electrode 116. The second region 112-2 included in the oxide semiconductor layer 112 has a carrier concentration lower than that of the first region 112-1. Therefore, the transistor 100 b forms the channel in the first region 112-1 of the oxide semiconductor layer 112. An electric current flows in an area, of the first region 112-1 of the oxide semiconductor layer 112, that is closer to the second region 112-2 (on the first gate electrode 116 side), and also in an area, of the first region 112-1, that is closer to the second insulating layer 106; namely, the electric current flows at both of the upper interface and the lower interface of the first region 112-1. Therefore, the field effect mobility of the transistor 100 b is increased. In addition, the threshold voltage of the transistor 100 b is suppressed from being fluctuated, and thus the transistor 100 b has a higher reliability because of stable electric characteristics.

One of the second gate electrode 104 and the first gate electrode 116 may be supplied with a constant potential (fixed potential) to act as a back gate. In the case where the transistor 100 b is of an n-channel type, one of the second gate electrode 104 and the first gate electrode 116 may be supplied with, for example, a potential lower than the source potential to act as a back gate electrode. As a result, the threshold voltage of the transistor 100 b is suppressed from being fluctuated. Alternatively, the second gate electrode 104 and the first gate electrode 116 of the transistor 100 b may be supplied with the same gate voltage to act as dual-gates. As a result, the transistor 100 b has an increased on-current and improved frequency characteristics.

In the transistor 100 b, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located on one surface of the oxide semiconductor layer 112 (surface facing the substrate 102). The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are in contact with the first region 112-1 of the oxide semiconductor layer 112. The first region 112-1 of the oxide semiconductor layer 112 has a conductivity higher than that of the second region 112-2. Therefore, the contact resistance between the first oxide conductive layer 108 a and the first region 112-1 of the oxide semiconductor layer 112, and the contact resistance between the second oxide conductive layer 108 b and the first region 112-1 of the oxide semiconductor layer 112, are decreased. In other words, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are in contact with the surface, of the oxide semiconductor layer 112, that faces the substrate 102, so that the contact resistance is decreased. The carrier concentration of the oxide conductive layer 108 is preferably 1×10²⁰/cm³ or higher, and more preferably 1×10²¹/cm³ or higher. The conductivity of the oxide conductive layer 108 is preferably 1×10² S/cm or higher, and more preferably 1×10³ S/cm or higher. The carrier concentration and the conductivity of the first region 112-1, of the oxide semiconductor layer 112, in contact with the oxide conductive layer 108 are both lower than those of the oxide conductive layer 108 by at least two digits. The carrier concentration and the conductivity of the second region 112-2, of the oxide semiconductor layer 112, in contact with the first region 112-1 of the oxide semiconductor layer 112 are both lower than those of the first region 112-1 by at least two digits. The carrier mobility of the second region 112-2 of the oxide semiconductor layer 112 is lower than that of the first region 112-1 of the oxide semiconductor layer 112.

3-3. Manufacturing Method

The method of manufacturing the transistor 100 b is the same as the method of manufacturing the transistor in embodiment 1 except that the second gate electrode 104 is formed on one surface of the substrate 102 and that the second insulating layer 106, the second conductive film 107 and the third conductive film 109 are formed on the second gate electrode 104. In this section, only the stage of forming the second gate electrode 104 on the substrate 102 will be described. The second gate electrode 104 may be formed of a metal material same as the material used to form the first gate electrode 116.

First, a first conductive film is formed on one surface of the substrate 102. Then, a resist mask is formed on the first conductive film by a photolithography step, and the second gate electrode 104 is formed by an etching step. The first conductive film is not limited to having any specific thickness, but is formed to have a thickness of, for example, about 100 nm to about 2000 nm. It is preferred that the second gate electrode 104 has tapered ends as seen in a cross-sectional view. The second gate electrode 104 has tapered ends and thus is covered with the second insulating layer 106 with certainty. Therefore, in the etching step of forming the second gate electrode 104, it is preferred to perform anisotropic etching on the first conductive film while chemically milling the resist mask; namely, to perform so-called taper etching. The resist mask that is left after the formation of the second gate electrode 104 is removed by use of a releasing solution or by an ashing process.

Like in embodiment 1, with the method of manufacturing the transistor 100 b in this embodiment, the first region 112-1 and the second region 112-2 of the oxide semiconductor layer 112 are formed continuously by using the same sputtering target and controlling the partial pressure of oxygen of the sputtering gas. This improves the productivity.

Embodiment 4

In embodiment 4, an example of display device including transistors having substantially the same structure as that of the transistor described in embodiment 3 will be described. Components that are the same as those in embodiments 1, 2 and 3 will bear the same reference signs, and the descriptions thereof will not be repeated.

4-1. Equivalent Circuit

FIG. 27 is an equivalent circuit diagram of a pixel 122 b included in the display device in this embodiment. The pixel 122 b includes a selection transistor 124 b, a driving transistor 126 b, the capacitance element 128, and the organic EL element 130. The selection transistor 124 b and the driving transistor 126 b each have substantially the same structure as that of the transistor 100 b described in embodiment 3. Namely, FIG. 27 shows the transistors of a dual-gate structure. The selection transistor 124 b includes a second gate electrode 104 b and the first gate electrode 116 b, and the driving transistor 126 b includes a second gate electrode 104 a the first gate electrode 116 a.

In this embodiment, the selection transistor 124 b and the driving transistor 126 b are each of an n-channel type. Gates of the selection transistor 124 b (the second gate electrode 104 b and the first gate electrode 116 b) are connected with the gate signal line 132 a. One of input/output terminals (source and drain) of the selection transistor 124 b is connected with the data signal line 134, and the other of the input/output terminals is connected with gates of the driving transistor 126 b (the second gate electrode 104 a and the first gate electrode 116 a). The gates of the driving transistor 126 b (the second gate electrode 104 a and the first gate electrode 116 a) are connected with the other of the input/output terminals of the selection transistor 124 b. A drain of the driving transistor 126 b is connected with the organic EL element 130, and a source of the driving transistor 126 b is connected with the second common line 136 b. One of the two terminals of the capacitance element 128 is connected with the other of the input/output terminals (source and drain) of the selection transistor 124 b. The other of the two terminals of the capacitance element 128 is connected with the first common line 136 a. The first common line 136 a and the second common line 136 b are supplied with, for example, a ground potential.

One of the two terminals of the organic EL element 130 is connected with the drain of the driving transistor 126 b, and the other of the two terminals of the organic EL element 130 is connected with the power supply line 138. The power supply line 138 is supplied with a power supply potential VDD, which is higher than the potential of the common line 136. In this embodiment, the terminal of the organic EL element 130 that is connected with the drain of the driving transistor 126 b is a cathode electrode, and the terminal of the organic EL element 130 that is connected with the power supply line 138 is an anode electrode.

4-2. Pixel Structure

FIG. 28 shows an example of planar structure of one pixel 122 b corresponding to the equivalent circuit shown in FIG. 27. FIG. 29A shows a cross-sectional structure taken along line A1-A2 in FIG. 28, and FIG. 29B shows a cross-sectional structure taken along line B1-B2 in FIG. 28. FIG. 29A shows a cross-sectional structure of the driving transistor 126 b and the organic EL element 130. FIG. 29B shows a cross-sectional structure of the selection transistor 124 b and the capacitance element 128. The following description will be made with reference to FIG. 28, FIG. 29A and FIG. 29B optionally. In the plan view of the pixel 122 b shown in FIG. 28, the structure of the organic EL element 130 is omitted.

4-2-1. Driving Transistor

The driving transistor 126 b has substantially the same structure as that of the transistor 100 b described in embodiment 3. Specifically, the driving transistor 126 b includes the second gate electrode 104 a, the second insulating layer 106, the first oxide semiconductor layer 112 a, the first insulating layer 114, and the first gate electrode 116 a, which are stacked. The second gate electrode 104 a is provided between the substrate 102 and the second insulating layer 106. The first gate electrode 116 a is provided on the top surface of the first insulating layer 114 (surface opposite to the surface facing the substrate 102).

The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are provided between the second insulating layer 106 and the first oxide semiconductor layer 112 a. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are provided in contact with the first oxide semiconductor layer 112 a to act as a source region and a drain region.

The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are provided to partially overlap the second gate electrode 104 a and the first gate electrode 116 a and to hold the second gate electrode 104 a and the first gate electrode 116 a from both of two sides as seen in a plan view. The second oxide conductive layer 108 b includes U-shaped curved patterns as seen in a plan view in at least a region thereof overlapping the second gate electrode 104 a and the first gate electrode 116 a. The first oxide conductive layer 108 a includes linear patterns extending inward of the U-shaped curved patterns of the second oxide conductive layer 108 b. The cross-sectional view of the transistor 100 b shown in FIG. 26 shows a unit structure from the first oxide conductive layer 108 a to the second oxide conductive layer 108 b. A cross-sectional view of the driving transistor 126 b shown in FIG. 28 taken along the second direction (D2 direction) includes a repetition of the cross-sectional view shown in FIG. 26. The second gate electrode 104 a is provided in the same layer structure as that of the first common line 136 a.

In the driving transistor 126 b in this embodiment, the oxide semiconductor layer 112 includes the first region 112-1 and the second region 112-2. The second region 112-2 has a carrier concentration lower than that of the first region 112-1. With such a structure, the driving transistor 126 b includes a channel in the first region 112-1, of the oxide semiconductor layer 112, separated from the first insulating layer 114. In the driving transistor 126 b in this embodiment, the oxide semiconductor layer 112 includes the second region 112-2 provided between the first region 112-1 and the first insulating layer 114. This improves the field effect mobility. In addition, the threshold voltage of the driving transistor 126 b is suppressed from being fluctuated, and thus the driving transistor 126 b has a higher reliability because of stable electric characteristics. The driving transistor 126 b has a dual-gate structure, and thus has an improved current driving capability. Therefore, the driving transistor 126 b provides a sufficient level of current to drive the organic EL element 130 even if the voltage of the second voltage 158 acting as an anode electrode is decreased. Even if the operating point of the organic EL element 130 is changed, the driving transistor 126 b provides constant current driving in accordance with the change in the operating point. The driving transistor 126 b adopts the dual-gate structure, and therefore, consumes less power. Therefore, the problem of heat generation, which is made conspicuous in a large-scale organic EL display device, is solved. This is effective to extend the life of the organic EL element.

4-2-2. Selection Transistor

The selection transistor 124 b has substantially the same structure as that of the transistor 100 b described in embodiment 3. Specifically, the selection transistor 124 b includes the second gate electrode 104 b, the second insulating layer 106, the second oxide semiconductor layer 112 b, the first insulating layer 114, and the first gate electrode 116 b, which are stacked. A channel of the second transistor 124 b is formed in a region where the second oxide semiconductor layer 112 b overlaps the second gate electrode 104 b and the first gate electrode 116 b.

The third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d are provided between the second insulating layer 106 and the second oxide semiconductor layer 112 b. The third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d are provided in contact with the second oxide semiconductor layer 112 b, and thus act as a source region and a drain region.

The third oxide conductive layer 108 c and the fourth oxide conductive layer 108 d are provided to partially overlap the second gate electrode 104 b and the first gate electrode 116 b and to hold the second gate electrode 104 b and the first gate electrode 116 b from both of two sides as seen in a plan view. The third oxide conductive layer 108 c includes a U-shaped curved pattern as seen in a plan view in at least a region thereof overlapping the second gate electrode 104 b and the first gate electrode 116 b. The fourth oxide conductive layer 108 d includes a linear pattern extending inward of the U-shaped curved pattern of the third oxide conductive layer 108 c. The cross-sectional view of the transistor 100 b shown in FIG. 26 shows a unit structure from the first oxide conductive layer 108 a to the second oxide conductive layer 108 b. A cross-sectional view of the selection transistor 124 shown in FIG. 28 taken along the second direction (D2 direction) includes a repetition of the cross-sectional view shown in FIG. 26. The second gate electrode 104 b is provided in the same layer structure as that of the first capacitance electrode 160 a.

4-3. Transistor Structure

As shown in FIG. 29A and FIG. 29B, the pixel 122 b in this embodiment has a structure in which the second electrode 158 covers the entire surfaces of the driving transistor 126 b and the selection transistor 124 b. The driving transistor 126 b and the selection transistor 124 b each have a bottom-contact dual-gate structure; specifically, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located in contact with the bottom surface of the oxide semiconductor layer 112 where a channel is formed, the second gate electrode 104 is located below the oxide semiconductor layer 112, and the first gate electrode 116 is located above the oxide semiconductor layer 112.

FIG. 30A and FIG. 30B each show a bottom-contact dual-gate structure of the transistor 100 b in this embodiment. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located in contact with the bottom surface of the oxide semiconductor layer 112, the first gate electrode 116 is located above the oxide semiconductor layer 112, and the second gate electrode 104 is located below the oxide semiconductor layer 112. The first region 112-1 of the oxide semiconductor layer 112 has a carrier concentration higher than that of the second region 112-2. Therefore, in the transistor 100 b in this embodiment, the electric current flows in an area, in the first region 112-1 of the oxide semiconductor layer 112, that is closer to the second region 112-2 (on the first gate electrode 116 side), so that the field effect mobility is improved. In addition, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are in contact with the first region 112-1, of the oxide semiconductor layer 112, having a high conductivity. Therefore, the contact resistance between the first oxide conductive layer 108 a and the first region 112-1 of the oxide semiconductor layer 112, and the contact resistance between the second oxide conductive layer 108 b and the first region 112-1 of the oxide semiconductor layer 112, are decreased.

As shown in this embodiment, with a structure in which the gate electrode is provided below and above the oxide semiconductor layer 112, the positive charges are suppressed from being accumulated on the back channel side of the oxide semiconductor layer 112. In this case, the second gate electrode 104 may be grounded to have a constant potential, or may be supplied with a voltage same as that of the first gate electrode 116, so that potential on the back channel side is stabilized.

FIG. 30A shows an embodiment of the transistor 100 b. In this embodiment, the transistor 100 b has a structure in which the second gate electrode 104 in a lower layer and the first gate electrode 116 in an upper layer each overlap both of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b, which correspond to the source and drain electrodes. The second gate electrode 104 having width W_(bottom) in a channel length direction overlaps each of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b by width W_(ov1). The first gate electrode 116 having width W_(top) in the channel length direction overlaps each of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b by width W_(ov2). Since the second gate electrode 104 and the first gate electrode 116 each partially overlap both of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b, the channel region in the oxide semiconductor layer 112 is substantially blocked against the external electric field. Therefore, even if the second electrode 158 is located to cover the entire surface of the transistor 100 b, the transistor 100 b is not influenced by the electric field of the second electrode 158. Thus, the threshold voltage of the transistor 100 b is prevented from being changed along with time.

FIG. 30B shows an embodiment of the transistor 100 b. In this embodiment, the transistor 100 b has a structure in which the first gate electrode 116 in an upper layer overlaps both of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b corresponding to the source and drain electrodes, and the second gate electrode 104 overlaps neither the first oxide conductive layer 108 a nor the second oxide conductive layer 108 b. The first gate electrode 116 having width W_(top) in the channel length direction overlaps each of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b by width W_(ov2). By contrast, width W_(bottom) of the second gate electrode 104 in the channel length direction is narrower than the interval between the first oxide conductive layer 108 a and the second oxide conductive layer 108 b, and the second gate electrode 104 is offset by width W_(off) from the first oxide conductive layer 108 a and the second oxide conductive layer 108 b. Since at least the first gate electrode 116 partially overlaps both of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b, the channel region in the oxide semiconductor layer 112 is substantially blocked against the electric field of the second electrode 158. Therefore, the threshold voltage of the transistor 100 b is prevented from being changed along with time. Specifically, the area size of the region where the first gate electrode 116 overlaps the oxide semiconductor layer 112 is larger than the area size of the region where the second gate electrode 104 overlaps the oxide semiconductor layer 112. For this reason, the influence of the charges that may be accumulated on the back channel side is blocked. In other words, the second gate electrode 104 and the first gate electrode 116 overlap each other as seen in a plan view, and the first gate electrode 116 covers the second gate electrode 104, and therefore, the influence of the charges that may be accumulated on the back channel side is blocked.

In consideration of the alignment precision of a photomask in a lithography step, it is preferable that width W_(top) of the first gate electrode 116 is larger than width W_(bottom) of the second gate electrode 104 (W_(top)>W_(bottom)). The width of the first gate electrode 116 is made larger than the width of the second gate electrode 104, so that there is a margin for the alignment precision of the photomask in the lithography step. Therefore, the channel region formed in the oxide semiconductor layer 112 is covered with the first gate electrode 116 with certainty.

4-4. Method of Manufacturing the Display Device

An example of method of manufacturing the display device 120 in an embodiment according to the present invention will be described. The method of manufacturing the display device 120 is the same as the method of manufacturing the display device in embodiment 1 except that the second gate electrode 104 is formed on one surface of the substrate 102 and that the second insulating layer 106, the second conductive film 107 and the third conductive film 109 are formed on the second gate electrode 104. In this section, only the stage of forming the second gate electrode 104 on the substrate 102 will be described.

The second gate electrode 104 a, 104b, the first capacitance electrode 160 a and the first common line 136 a are formed on one surface of the substrate 102. The first common line 136 a and the first capacitance electrode 160 a are formed of the same conductive film as the second gate electrode 104 a, 104b. Therefore, the second gate electrode 104 a and the gate signal line 132 a are formed as one continuous pattern formed of a conductive film in the same layer. The first common line 136 a and the first capacitance electrode 160 a are formed as one continuous pattern formed of a conductive film in the same layer.

Like in embodiment 1, with the method of manufacturing the display device 120 in this embodiment, the multi-gradation photomask 201 is used to decrease the number of photomasks required to manufacture the display device 120. FIG. 31A and FIG. 31B each show a pattern of a photomask usable to manufacture the display device 120, more specifically, a part of the photomask in the vicinity of the capacitance element 128 and the selection transistor 124. FIG. 31A shows the multi-gradation photomask usable to form the second gate electrode 104 b, the first capacitance electrode 160 a and the first common line 136 a. By use of the multi-gradation photomask 201 having such a pattern, a plurality of patterns (the second gate electrode 104 a, 104 b, the second capacitance electrode 160 a, the first common line 136 a) are formed by one cycle of exposure. FIG. 31B shows a photomask usable to form the third contact hole 117 c that connects the second gate electrode 104 b and the first gate electrode 116. By using such a photomask in addition to the photomask used in the method for manufacturing the display device 120 according to the first embodiment, the reliability of the display device 120 can be improved and the power consumption can be reduced.

Like in embodiment 1, with the method of manufacturing the transistor 100 b in this embodiment, the first region 112-1 and the second region 112-2 of the oxide semiconductor layer 112 are formed continuously by using the same sputtering target and controlling the partial pressure of oxygen of the sputtering gas. This improves the productivity.

In this embodiment, both of the selection transistor 124 b and the driving transistor 126 b are of a dual gate type. The present invention is not limited to this. For example, the selection transistor 124 b or the driving transistor 126 b may be a top gate type transistor with no second gate electrode 104 b. The pixel circuit is not limited to having a configuration shown in FIG. 27. The transistors and the organic EL element in this embodiment are applicable to a pixel circuit including three or more transistors for one pixel.

Embodiment 5

In embodiment 5, a transistor having a structure different from that in embodiment 1 will be described. In embodiment 5, unlike in embodiment 1, the first line 110 a is located between the first oxide conductive layer 108 a and the second insulating layer 106, and the second line 110 b is located between the second oxide conductive layer 108 b and the second insulating layer 106. Components that are the same as those in embodiment 1 will bear the same reference signs, and the descriptions thereof will not be repeated.

5-1. Transistor Structure

FIG. 32 is a cross-sectional view showing a structure of a transistor 100 c in an embodiment according to the present invention. The transistor 100 c includes the second insulating layer 106, the oxide semiconductor layer 112, the first insulating layer 114, and the first gate electrode 116, which are provided on the substrate 102 having an insulating surface.

The first gate electrode 116 is located to face one surface of the oxide semiconductor layer 112 (surface opposite to a surface facing the substrate 102). The first insulating layer 114 is located between the oxide semiconductor layer 112 and the first gate electrode 116. The first gate electrode 116 and the oxide semiconductor layer 112 are located to partially overlap each other while having the first insulating layer 114 between the first gate electrode 116 and the oxide semiconductor layer 112. In the transistor 100 c, a channel is formed in a region where the oxide semiconductor layer 112 overlaps the first gate electrode 116. The first insulating layer 114 acts as a gate insulating film in a region where the oxide semiconductor layer 112 and the first gate electrode 116 overlap each other.

The second insulating layer 106 is located between the oxide semiconductor layer 112 and the substrate 102. A first oxide conductive layer 108 a and a second oxide conductive layer 108 b are located between the oxide semiconductor layer 112 and the second insulating layer 106. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are provided in contact with the oxide semiconductor layer 112. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located such that one end part of the first oxide conductive layer 108 a and one end part of the second oxide conductive layer 108 b overlap the first gate electrode 116. One of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b acts as a source region, and the other of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b acts as a drain region. In the structure shown in FIG. 32, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located such that one end part of the first oxide conductive layer 108 a and one end part of the second oxide conductive layer 108 b overlap the first gate electrode 116. Therefore, the oxide semiconductor layer 112 does not include any offset region (region having a high resistance). Thus, the level of on-current is increased.

A first line 110 a is provided in contact with the first oxide conductive layer 108 a, and a second line 110 b is provided in contact with the second oxide conductive layer 108 b. The first line 110 a is located between the first oxide conductive layer 108 a and the second insulating layer 106, and the second line 110 b is located between the second oxide conductive layer 108 b and the second insulating layer 106. According to the transistor 100 c in this embodiment, the oxide semiconductor layer 112 is not in direct contact with the first line 110 a and the second line 110 b, so that the channel region is prevented from being contaminated with a metal used as a material of the first line 110 a and the second line 110 b.

Embodiment 6

In embodiment 6, a transistor having a structure different from that in embodiment 5 will be described. In embodiment 6, unlike in embodiment 5, the second gate electrode 104 is located to face one surface of the oxide semiconductor layer 112 (surface facing the substrate 102). Components that are the same as those in embodiments 1 to 5 will bear the same reference signs, and the descriptions thereof will not be repeated.

6-1. Transistor Structure

FIG. 33 is a cross-sectional view showing a structure of a transistor 100 d in an embodiment according to the present invention. The transistor 100 d includes the second gate electrode 104, the second insulating layer 106, the oxide semiconductor layer 112, the first insulating layer 114, and the first gate electrode 116, which are provided on the substrate 102 having an insulating surface.

The second gate electrode 104 is located to face one surface of the oxide semiconductor layer 112 (surface facing the substrate 102). The second insulating layer 106 is located between the oxide semiconductor layer 112 and the second gate electrode 104. The first gate electrode 116 is located to face the other surface of the oxide semiconductor layer 112 (surface opposite to the surface facing the substrate 102). The first insulating layer 114 is located between the oxide semiconductor layer 112 and the first gate electrode 116. The second gate electrode 104 and the first gate electrode 116 are located to partially overlap each other while having the second insulating layer 106, the oxide semiconductor layer 112 and the first insulating layer 114 between the second gate electrode 104 and the first gate electrode 116. In the transistor 100 b, a channel is formed in a region where the oxide semiconductor layer 112 overlaps the second gate electrode 104 and the first gate electrode 116. The second insulating layer 106 acts as a gate insulating film in a region where the oxide semiconductor layer 112 and the second gate electrode 104 overlap each other. The first insulating layer 114 acts as a gate insulating film in a region where the oxide semiconductor layer 112 and the first gate electrode 116 overlap each other.

The second insulating layer 106 is located between the oxide semiconductor layer 112 and the second gate electrode 104. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located between the oxide semiconductor layer 112 and the second insulating layer 106. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are provided in contact with the oxide semiconductor layer 112. The first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located such that one end part of the first oxide conductive layer 108 a and one end part of the second oxide conductive layer 108 b overlap each of the second gate electrode 104 and the first gate electrode 116. One of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b acts as a source region, and the other of the first oxide conductive layer 108 a and the second oxide conductive layer 108 b acts as a drain region. With the structure shown in FIG. 26, the first oxide conductive layer 108 a and the second oxide conductive layer 108 b are located such that one end part of the first oxide conductive layer 108 a and one end part of the second oxide conductive layer 108 b overlap each of the second gate electrode 104 and the first gate electrode 116. Therefore, the oxide semiconductor layer 112 does not include any offset region (region having a high resistance). Thus, the level of on-current is increased.

A first line 110 a is provided in contact with the first oxide conductive layer 108 a, and a second line 110 b is provided in contact with the second oxide conductive layer 108 b. The first line 110 a is located between the first oxide conductive layer 108 a and the second insulating layer 106, and the second line 110 b is located between the second oxide conductive layer 108 b and the second insulating layer 106. According to the transistor 100 d in this embodiment, the oxide semiconductor layer 112 is not in direct contact with the first line 110 a and the second line 110 b, so that the channel region is prevented from being contaminated with a metal used as a material of the first line 110 a and the second line 110 b.

The present invention is not limited to the above-described embodiments, and can be modified as appropriate in a range not deviating from the gist of the invention. Also, the embodiments can be combined as appropriate. 

What is claimed is:
 1. A transistor comprising: an oxide semiconductor layer on a substrate, the oxide semiconductor layer including a first layer and a second layer; a first gate electrode including a region overlapping the oxide semiconductor layer; a first insulating layer between the first gate electrode and the oxide semiconductor layer; and a first oxide conductive layer and a second oxide conductive layer each including a region in contact with the oxide semiconductor layer, wherein the first layer of the oxide semiconductor layer and the second layer of the oxide semiconductor layer include a region overlapping each other, the first layer of the oxide semiconductor layer is arranged closer to the top surface of the substrate than the second layer of the oxide semiconductor layer, and wherein a band gap of the second layer of the oxide semiconductor layer is wider than a band gap of the first layer of the oxide semiconductor layer.
 2. The transistor according to claim 1, wherein the first gate electrode is arranged to face a surface of the oxide semiconductor layer opposite to a top surface of the substrate.
 3. The transistor according to claim 1, wherein the first oxide conductive layer and the second oxide conductive layer are arranged between the oxide semiconductor layer and the substrate.
 4. The transistor according to claim 1, further comprising a second insulating layer between the substrate and the oxide semiconductor layer, the second insulating layer including a region in contact with the first layer of the oxide semiconductor layer.
 5. The transistor according to claim 1, wherein an end part of the first oxide conductive layer and an end part of the second oxide conductive layer each overlap the first gate electrode.
 6. The transistor according to claim 2, further comprising a second insulating layer between the substrate and the oxide semiconductor layer; and a second gate electrode including a region overlapping the oxide semiconductor layer and the first gate electrode, the second gate electrode being arranged between the second insulating layer and the substrate.
 7. The transistor according to claim 6, wherein a width of the first gate electrode in a channel length direction is larger than a width of the second gate electrode in the channel length direction.
 8. The transistor according to claim 1, wherein a conductivity of the first layer of the oxide semiconductor layer is higher than a conductivity of the second layer of the oxide semiconductor layer.
 9. The transistor according to claim 1, wherein a band gap of the first layer of the oxide semiconductor layer is smaller than a band gap of the second layer of the oxide semiconductor layer, and a work function of the first layer of the oxide semiconductor layer is larger than a work function of the second layer of the oxide semiconductor layer.
 10. The transistor according to claim 1, wherein a carrier concentration of the second layer of the oxide semiconductor layer is lower than a carrier concentration of the first layer of the oxide semiconductor layer.
 11. The transistor according to claim 1, wherein a carrier mobility of the first layer of the oxide semiconductor layer is higher than a carrier mobility of the second layer of the oxide semiconductor layer.
 12. The transistor according to claim 1, wherein a crystallization ratio of the first layer of the oxide semiconductor layer is lower than a crystallization ratio of the second layer of the oxide semiconductor layer.
 13. The transistor according to claim 1, wherein a film thickness of the first layer of the oxide semiconductor layer is larger than a film thickness of the second layer of the oxide semiconductor layer.
 14. The transistor according to claim 1, wherein the first insulating layer includes a silicon oxide film in contact with the oxide semiconductor layer and a silicon nitride film in contact with the silicon oxide film.
 15. The transistor according to claim 1, wherein a material of the first insulating layer is a hydrogen-free silicon-based material.
 16. The transistor according to claim 1, wherein a material of the second layer of the oxide semiconductor layer is an oxide material containing amorphous Ga₂O₃.
 17. The transistor according to claim 1, wherein the first oxide conductive layer and the second oxide conductive layer each have a region overlapping with the first gate electrode, the second oxide conductive layer has a pattern curved in a U shape in a planer view at least in a region overlapping with the first gate electrode, and the first oxide conductive layer has a linear pattern extending inward of the pattern curved in the U shape in the planer view.
 18. A transistor comprising: an oxide semiconductor layer on a substrate, the oxide semiconductor layer having a first surface on a side of the substrate and a second surface opposite to the first surface; a first gate electrode including a region overlapping the oxide semiconductor layer; a first insulating layer between the first gate electrode and the oxide semiconductor layer; and a first oxide conductive layer and a second oxide conductive layer each including a region in contact with the first surface of the oxide semiconductor layer; wherein a band gap of the second layer of the oxide semiconductor layer is wider than a band gap of the first layer of the oxide semiconductor layer.
 19. The transistor according to claim 18, wherein the first gate electrode is arranged to face the second surface of the oxide semiconductor layer.
 20. The transistor according to claim 1, wherein the first oxide conductive layer and the second oxide conductive layer are arranged between the oxide semiconductor layer and the substrate. 